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A rudimentary Atari BASIC program ready to run
|Stable release||Revision C / 1983|
|Operating system||Atari 400/800/XL/XE|
|License||Copyright © 1979 Atari Inc. Proprietary|
ATARI BASIC is a BASIC interpreter for the Atari 8-bit family of 6502-based home computers. The interpreter originally shipped on an 8 KB cartridge; on later XL/XE model computers it was built in, with an option to disable it, and started when the machines were booted with no other cartridges in place. The complete commented source code and design specifications of ATARI BASIC were published as a book in 1983. This marked the first time source code was made available for a commercial language.
Nomenclature and symbols
In the nomenclature of the time when these machines were designed, "K" was taken to mean one kilobyte, so that is how it is expressed here. Similarly, further, on the family of processors use by Atari machines, in the assembly langage "
$" introduced a hexadecimal number, or it was suffixed subscripted with its radix, so, for example, "Template:Number to word" is "
$80", or "
8016". If a number is expressed with no radix, decimal (10) is assumed, and a leading 0 does not imply octal. These are used instead of more modern conventions because they appear as such in many of the sources and will be seen often by those using simulators an so forth.
The machines that would become the Atari 8-bit family had originally been developed as second-generation games consoles intended to replace the Atari 2600. Ray Kassar, the new president of Atari, decided to challenge Apple Computer by building a home computer instead. This meant Atari needed the BASIC programming language, then the standard language for home computers.
Atari did what many of the other home computer companies did: they purchased the source code to the MOS 6502 version of Microsoft 8K BASIC, intending to port it to run on their new machines. But the name was something of a misnomer, as the 8K referred to its original size on the Intel 8080's instruction set. The 8080 has more memory registers and addressing modeđ than the 6502, that is to say, it can store more little bits of information on the chip itself without having to ask for it from memory, and if it does, it has more eloquent ways to ask for it. The 6502 is far simpler, and in a sense faster, but when something complicated must be done it has constantly to shuffle from memory to its own registers or between them and back and so on, all of which require instructions to the processor which themselves take memory.. So the Microsoft 8K Basic, when translate to the 6502 instruction set, ended up at around 9K. Of course, it is to be expected that any good programmer would write to target a given machine, the 8080 in this case, and so the 6502 implementation got all of the flaws of 8080 instruction set, but none of the benefits of its own.
So Atari felt that they needed to expand the language somewhat to add better support for the specific hardware features of their computers, similar to what Apple had done with their Applesoft BASIC. This increased the size again, from 9K to around 11K. Atari had designed their ROM layout in 8K blocks, and paring down the code from 11K to 8K turned out to be a significant problem. Adding to the problem was the fact that the 6502 code supplied by Microsoft was undocumented.
Six months later, they were almost ready. But Atari had a deadline with the Consumer Electronics Show (CES) approaching and decided to ask for help.
|The examples and perspective in this article may not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page. (December 2009)|
In September 1978 Atari asked Shepardson Microsystems (SMI) to bid on completing BASIC. Shepardson had written a number of programs for the Apple II family, which used the same 6502 processor, and were in the middle of finishing a new BASIC for the Cromemco S-100 bus machines (Cromemco 32K Structured BASIC). SMI examined the existing work and decided it was too difficult to continue paring it down; instead they recommended developing a completely new version that would be easier to fit into 8K. Atari accepted the proposal, and when the specifications were finalized in October 1978, Paul Laughton and Kathleen O'Brien began work on the new language.
The result was a rather different version of BASIC, known as ATARI BASIC. In particular, the new BASIC dealt with character strings more like Data General's BASIC than Microsoft's, which used strings similar to those from DEC BASIC.
The contract specified a delivery date on or before April 6, 1979 and this also included a File Manager System (later known as DOS 1.0). Atari's plans were to take an early 8K version of Microsoft BASIC to the 1979 CES and then switch to the new Atari BASIC for production. Because of a bonus clause in the contract, development proceeded quickly and an 8K cartridge was available just before the release of the machines. Because ATARI BASIC was delivered before Microsoft BASIC, Atari took it with them to the CES.
Shepardson's programmers found problems during the first review and managed to fix some of them, but Atari had already committed ATARI BASIC to manufacturing, and the technology of the time did not permit changes. So it was manufactured with known bugs, and became known (as a retronym) Revision A.
- Revision A – First Atari BASIC cartridge. 8K ROM.
- Revision B – Fixed all of the major software bugs in Revision A, but introduced a leaking memory bug. Found built-in on the 600XL and early 800XLs. Never supplied on cartridges.
- Revision C – Eliminated memory leak in Revision B. Found on later 800XLs, the 800XLF, XEGS and all XE computers. Limited cartridge production run.
A BASIC programmer can find out the version by examining a well-known location in memory. Entering the command
PRINT PEEK(43234) at the READY prompt will give a result of
162 for Revision A,
96 for Revision B, and
234 for Revision C.
Program editingsyntax errors. If a problem is found it re-prints the line, highlighting the text near the error. This makes catching errors on the Atari much easier than on other editors, as most BASICs will not display most errors until the program was later
RUN, when the new line would no longer be fresh in the author's mind.
When not running a program, Atari BASIC is in immediate mode, where lines can be entered (with a line number) to add a line in the program or modify an existing one, or commands can be entered (without a line number) that are executed immediately. Unlike most other BASICs, however, Atari BASIC allows all commands to be executed in both modes. For instance most BASICs only allow
LIST to be used in immediate mode, while Atari BASIC also allows it to be used inside a program. This was sometimes used as part of a way to produce self modifying code.
Program lines ("logial lines") can be up to three screen lines ("physical lines") of 40 characters, so 120 characters total. The cursor can be moved freely in these lines, unlike in other BASICs where to get "up" a line one has to continuously scroll leftwards until the cursor is wrapped at the left margin (and similarly to go down when wrapping at the right margin) – though that works too, except the cursor when wrapping left to right or right to left does not move up or down a line. The OS handles tracking whether a physical line flowed to the next on the same logical line; the three-line limit is fairly arbitrary but keeps lines below 128 characters and so probably reduces the chances of buffer overflow.
It is actually possible to move the cursor anywhere on the screen, and it will wrap on all sides. Hitting "enter" will submit that (logical) line to the tokenizer. So, in the example pictured above (with "
PRUNT", all that will be required for the fix is to move the cursor over the "
U", type "
I" (the editor only has an overwrite mode) and then hit Enter. This is quite a frequent editing technique for, say, renumbering lines, since there is no built-in renumbering facility but quickly one can learn to, say, overwrite the numbers on a set of lines then just hit return repeatedly to enter them back into the program. Indeed, a slightly cryptic but essentially simple idiom allows this to be done by the program itself as it is running, producing self-modifying code. This is not an artifact or cheat around the system but inherent in the combined behavior of the editor and tokenizer.
The Atari variation on ASCII, called ATASCII, has 128 (8016) characters mostly corresponding to ASCII but with a few exceptions. All characters have printable forms unlike ASCII where codes 0-31 (0-1F16) were "control codes" that performed special functions such as requesting a paper feed or ringing an attention bell. Characters 128-255 (8016-FF16) were displayed as the inverse video of characters 0-127 (0016-7F16). Variable names had to be composed of upper-case alphabetic (65-90, 4116-5A16) and numeric (48-57, 3016-3916) characters, starting with an alphabetical character, and for strings terminating with a dollar sign (36, 2416).
The character set has a full ensemble of lower case characters and some graphics characters, but it is frequent to see programs mostly in upper case. This may be simply because of the conventions of the time, previous machines not having lower case; or that some devices such as daisy wheel printers may not have lower case, or that the lower-case font was not very attractive, in particular struggling to squeeze ascenders and descenders into the 8×8 fixed grid used to define each glyph, or the 8×7 fonts used on many dot matrix printers such as Atari's own.
On Revision C ROMs there is an alternative font including characters with diacritics, intended for European users, but this is rarely used. Adding user-defined fonts is relatively easy and each took 1 kilobyte; a one-byte is used to indicate the start page of a font (though only one could be used at a time without machine code display list interrupts to change the font midway down the screen). Similarly on later GTIA graphics processors, a little-known 8×10 font mode exists, where two lines top or bottom of a character are assumed blank, thus keeping the actual glyphs to 8×8 but allowing ascenders or descenders without these characters touching those above or below; this is very rarely used, partly because dot matrix printers can not easily support it. The ease of implementing other fonts means many are freely available, with font editors and so forth too.
Like most BASIC interpreters, Atari BASIC uses a token structure to handle lexical processing for better performance and reduced memory size. The tokenizer converts lines using a small buffer in memory, and the program is stored as a parse tree. The token output buffer (addressed by a pointer at LOMEM – 80, 8116) is 1 page (256 bytes) long, and any tokenized statement that is larger than the buffer will generate a BASIC error (14 – line too long). Indeed, the syntax checking described in the "Program editing" section is a side-effect of converting each line into a tokenized form before it is stored. Sinclair BASIC uses a similar approach, though it varies between models.
The output from the tokenizer is then moved into more permanent storage in various locations in memory. A set of pointers (addresses) indicates these locations: variables are stored in the variable name table (pointed to at VNTP – 82, 8316) and the values are stored in the variable value table (pointed to at VVTP – 86, 8716). Strings have their own area (pointed to at STARP – 8C, 8D16) as does the runtime stack (pointed to at RUNSTK – 8E, 8F16) used to store the line numbers of looping statements (
FOR...NEXT) and subroutines (
GOSUB...RETURN). Finally, the end of BASIC memory usage is indicated by an address stored at MEMTOP – 90, 9116) pointer.
By indirecting the variable names in this way, a reference to a variable needs only two bytes to address its entry into the appropriate table; the whole name does not need to be stored each time. This also makes variable renaming relatively trivial if the program is in storage, as it is simply a case of changing the single instance of its name in the table and the only difficulty is if the name changes length (and even then, only if it gets longer): indeed, obfuscated code can be produced for a finished program by renaming variables in the name tables – possibly all to the same name. This doesn't confuse the interpreter since internally it is using the index values not the names. Of course, new code will be difficult to add because the tokenizer has to search the name table to find a variable's index, and can get confused if names were not unique (though it is OK to have names in both the "string" and "variable" namespaces, e.g.
HELLO = 10 and
HELLO$ = "WORLD", because they have separate tables.)
ATARI BASIC uses a unique way to recognise abbreviated reserved words. In Microsoft BASIC there are a few predefined short forms, (like
REM). Atari BASIC allows any keyword to be abbreviated using a period, at any point in writing it. So
L. will be expanded to
LIST, as will
LI. and (redundantly)
LIS.. To expand an abbreviation the tokenizer will search through its list of reserved words and find the first that matches the portion supplied. To improve the probability of the programmer correctly guessing an abbreviation, and to save typing, the list of reserved words is sorted to place the more-commonly used commands first.
REM is at the very top, and can be typed in just as
.. This also speeds lexical analysis generally, since although the time to search is in theory proportional to the length of the list, in practice it will find common keywords very quickly, to the extent that good programmers knew when a line was in error even before the parser said so, as the time taken to return the error after exhaustively searching the list in itself gave an indication that something was wrong.
So whereas Microsoft BASIC uses separate tokens for its few short forms, ATARI BASIC has only one token for each reserved word – when the program is later
LISTed it will always write out the full words (since only one token represents all possible forms, it can do no other). There are two exceptions to this:
LET has a synonym which is the empty string (so
10 LET A = 10 and
10 A = 10 mean the same thing). These are separate tokens, and so will remain as such in the program listing.
Some other contemporary BASICs have variants of keywords that include spaces (for example
GO TO). Atari BASIC does not. The main exception here is keywords for communicating with peripherals (see the "Input/Output" section, below) such as
OPEN # and
PRINT #; it rarely occurs to many programmers that the "
#" is actually part of the tokenized keyword and not a separate symbol; an that for example "
PRINT #0" are the very same thing, just presented differently. It may be that the BASIC programmers kept the form
# to conform with other BASICs (The syntax derives from Fortran), though it is entirely unnecessary, and probably a hindrance, for tokenizing, and is not used in other languages for the Atari 8-bit family.
Expanding tokens in the listing can cause problems when editing. The Atari line input buffer is three lines (120 characters); up to three "physical lines" make one "logical line". After that a new "logical line" is automatically created. This doesn't matter much for output but it does for input, because the operating system will not return characters to the tokenizer after the third line, treating them as the start of a new "logical line". (The operating system keeps track of the mapping between physical and logical lines as they are inserted and deleted.) But using abbreviations when typing in a line can result, once they have been expanded on output, to a line that is onger than three lines and, a more minor concern, some white space can be omitted on input, so for example
PRINT"HELLO" will be listed as
PRINT "HELLO", one character longer. If one then wants to edit the line, now split across two logical lines, one needs to replace the expanded commands back with their abbreviations to be able to submit them back to the tokenizer. The moral of this is, generally, don't try to squeeze more out of a line than is reasonable, or, in the alternate, obfuscate variables by
Also, literal line numbers in statements such as
GOTO are calculated at runtime using the same math routines as other BASIC functions. This calculation allows subroutines to be referred to by variables, for instance
GOTO EXITOUT, as long as one remembers to initialize
EXITOUT. This is much more useful than it might sound; tokenized variable values are stored in a six-byte format only once in memory (in the variable value table, at VVTP), and the GOTO target is to by just the two-bytes index into this VVT, whereas explicit line numbers take up the six bytes needed for a floating point (FP) number every time they appear in the program. (The design choice to store them as FP is discussed later.) If many parts of a program jump to a single line number, such as a subroutine, which is fairly common in BASIC, it can save memory to replace them with variables, and it also means those subroutines can be renumbered since only the variable value needs to be changed. For this reason it is common to see, at the start of an ATARI program, a list of assignments of line numbers to variables..
Atari BASIC differs dramatically from Microsoft-style BASICs in the way it handles strings. In BASICs following the Microsoft model, strings are special types that allow for variable length and various operations. Atari BASIC has no strings of this sort, instead using arrays of characters, rather like Fortran. This allowed the BASIC language programmers to remove all the special-purpose code needed for handling dynamic resizing of strings, reusing instead the code already being used to handle arrays of numbers. A string is allocated a maximum size using the
DIM statement, although its actual length can vary at runtime from 0 to this maxium size.
Of course, strings are not used by end programmers in the same way as arrays of numbers – at least not normally – so Atari BASIC also includes a selection of commands for "slicing" up arrays.
A$ referred to the entire string, whereas
A$(4,6) "sliced" out the three characters 4, 5 and 6. In theory, this was a more elegant solution than Microsoft BASIC's
RIGHT$ solution, as this syntax replaces three separate commands with a single one.
Although this simplification reduces the size of Atari BASIC and offered some theoretical performance benefits, it also makes it much more difficult to port BASIC programs onto the Atari, arguably more so than any other difference. When it was the norm for programs to be provided as listings in magazines, users would have to scan them for instances of
LEFT$, and so on, and replace them with slicing commands. Also, because strings were allocated a fixed size it generally meant that this size would be guessed and pessimized to a guesstimate of their likely maximum size.
A common trick is to assign
MYSTRING$(2), this copies the same character from the first in the string into every position in the string, relying on the order of copying used in the Atari BASIC array copy routine – it is essentially undefined behavior.
Strings in Atari BASIC cannot themselves be members of arrays, so arrays of strings have to be implemented by the programmer. Strings can move around in memory, so it is not generally possible for example to store their memory addresses in an array. For short strings of approximately the same length, instead an array is generally built using padding so that the strings so they are all the same length and the nth string in the array is n×l characters into it, where l is the length of the string. According to Bill Wilkinson, the chief programmer at SMI, it was unfeasible to implement string arrays with strings that were larger than one page (256 characters).
The Atari OS includes a subsystem for peripheral device input/output (I/O) known as CIO (Central Input/Output). All I/O went through a central point of entry (E45C16) passing the address of an I/O Control Block (IOCB), a 16-byte structure that defines which device was meant, and what kind of operation (read, write, seek etc). There are 8 such IOCBs, allocated at fixed locations in page 3 of memory from
Most progams therefore can be written independently of what device they might use, as they all conform to a common interface – this was very rare on home computers when ATARI BASIC was first made. virtual devices such as the screen,
S: and the editor,
E:<code> did have special operations, for example to draw graphics or to ask for line input (in fact <code>E: was pretty much a combination of
S: and the keyboard,
K:), but these were done in a uniform way and new device drivers could be written fairly easily that would automatically be available to ATARI BASIC and indeed any other program using the Atari OS, for example to provide support for new hardware devices such as mouse pointers, or software devices such as an 80-column display (using typically a 4×8 pixel font). Existing drivers could be supplanted or augmented by new ones since the driver table was searched newest-to-oldest, so a replacement
E:, for example could displace the one in ROM to provide an 80-column display, or to piggy-back on it to generate a checksum whenever a line was returned – this technique is used for some of the program listing checkers that provide a checksum for each line.
CIO access in BASIC
Atari BASIC supports CIO access with reserved words
OPEN #, CLOSE #, PRINT #, INPUT #, GET #, PUT #, NOTE #, POINT # and
XIO #. There were routines in the OS, for example for graphics fill and draw, that were not available as specific BASIC keywords, but could be obtained through
XIO. Since they were already provided in the OS, one can only assume the BASIC programmers ran out of time or space to add them to the BASIC tokenizer: they are available in variants such as Turbo-BASIC XL, and in other languages.
Up to eight IOCBs can be in use at a time, numbered 0 through 7 (0 was, by default, the editor
E:). The BASIC statement
OPEN # was used to prepare a device for I/O access:
REM Opens the cassette device on channel 1 for reading in BASIC OPEN #1,4,0,"C:MYPROG.DAT"
OPEN # means "ensure channel 1 is free" (an error otherwise results), call the
C: driver to prepare the device (this will set the cassette tape spools onto tension and advance the heads keeping the cassette tape player "paused"; the
4<code> means "for read" (other codes were <code>8 for write,
12 = 8 + 4 for "read-and-write", and so forth), and the third number provides extra auxiliary information, here not used and set by convention to 0. The
C:MYPROG.DAT is the name of the device and the filename, as it happens, files on cassette were not named by this device. The string gives the device name and optionally a filename. Physical devices can have numbers (mainly disks, printers and serial devices), so "P1:" might be the plotter and "P2:" the daisy-wheel printer, or "D1:" may be one disk drive and "D2:" another, "R1:" may be a modem and "R2:" an oscilloscope (R for RS-232, provided by an add-on interface and not built into the OS), and so on; if not present, 1 is assumed.
Reserved IOCBs in ATARI BASIC
ATARI BASIC disallows access to IOCB 0 (the editor,
E:) and reserves IOCB 7 for printing and cassette operations using the built-in commands
LPRINT, SAVE, LOAD, CSAVE, CLOAD, though there is nothing to stop printers or the cassette being used on other channels too. IOCB 6 is used for accessing the graphics screen device (
S:) for drawing lines, filling shapes and so on.
LOAD output the compact tokenized form of the BASIC program,
ENTER output and input the text source, just as if they were being sent to or from the editor.
For the other CIO functions, ATARI BASIC uses the
XIO statement. This essentially just primed an IOCB and called the CIO entry point; any of the other commands could be achieved with
XIO, but its form was not very friendly for BASIC users. This was used for special functions included screen functions and serial (RS-232) functions (on
R:) as well as disk operations like format or deleting a file.
It could be argued that some statements such as
LPRINT<code>, <code>CLOAD<code> and <code>CSAVE<code> are redundant; the ROM space used by for the names of these routines could perhaps be better used to give access to other things already provided by the OS, and it puts IOCB 7 out of bounds (that being said, very few programs need to use more than a few IOCBs).
I/O routines returned error codes of 128-255 (8016-FF16) via the processor's Y register and setting the carry flag of the processor. Setting the carry flag is a neat trick since the caller can immediately branch-on-carry (BRC) to an error routine, a brief, quick and relocatable 6502 instruction (2 bytes, 2 cycles), without having to test Y for the (we hope) normal case where there is no error.
As with other aspects of the CIO, error codes were common across devices but could be extended for particular devices. Error handlers could thus be written quite generically, to fail gracefully, maybe put out a message, ask the user whether to retry, propagate the error, and so on.
There were no user-friendly messages for standard error codes in the OS itself. They would be interpreted by the application.
Atari BASIC (and other languages) thus had the freedom to return error codes less than 128, and these meant different things in different languages. There was nothing to stop a perverse implementer using error codes of 128 or above, but no incentive to do so.
The Atari hardware had (for its time) quite a sophisticated graphics system, stemming from its basis in video games consoles. Unlike many other home computers of the time, the graphics "mode" – the size of pixels and the number of colors that could be displayed – was not fixed but was described on a line-by-line basis in a small microcontrol language to create a display list. Each entry in this list described one or more lines on the TV display, top-to-bottom. A dedicated graphics microprocessor, "ANTIC", read these out during the horizontal blanking interval to determine how to display the next TV line. Lines could be narrow (256 pixels wide at highest resolution), normal (320 pixels) or wide (384 pixels). Characters were 8×8 and one display list entry would thus describe 8 TV lines, the ANTIC would keeping a counter (shift register) of which line in the font to read the pixel data from for each of those lines, thus only requiring one entry for the whole 8 lines in the display. Similarly for larger, coarser graphics modes, the ANTIC chip kept track of the information so that it would display the data on more than one TV line.
Later (XL and XE) machines had an additional graphics processor called CTIA (later GTIA) offering additional modes, providing additional graphics modes, but for the purposes of discussing Atari BASIC these do not affect the basic description as they were programmed much the same way.
Sprites (Player/Missile Graphics)
A hardware sprite (computer graphics) system was also handled by ANTIC. A sprite was essentially a character 8 pixels wide and 256 TV lines long (note this did not vary with the graphics mode), in one color. (The sprite actually extended the whole height of the display including the screen border but the top and bottom were just a solid color.) It could be considered an extremely tallcharacter in a font. ANTIC would automatically overlay the sprite "stripe" at a horizontal position, so that where there was a 0 bit in the sprite definition the normal graphics pixel color would be displayed, but where there was a 1 bit the sprite color would be displayed. Moving the sprite vertically was achieved by block moving (or rotating) its definition in memory, which was reasonably easy in machine language although the 6502 processor has no built-in block memory move instruction.
Although these sprites had quite severe limitions because of their width and color restrictions, careful use of them could make graphics programming, particularly games, significantly simpler since it reduced the need to manipulate display memory for fast-moving objects (such as the "player" and his weapons in a shoot 'em up fame (in fact the official name of the sprite system was "Player/Missile Graphics"). In fact There were four sprites, but they could be overlapped making multi-colored sprites, at the expense of reducing the number of sprites available for other uses.
The ANTIC microprocessor had many "registers" by which colors could be set, and the horizontal position (and enabling) of the sprites, and so forth. For example, for a four-colour graphics mode, only four colours could be displayed, but these could be chosen from any of the 256 in the physical color palette. For 2-color modes there was actually only one color, which could be displayed at two levels of luminance. Data in memory described the pixel colors as "logical colors", for example for a 4-color mode two bits of a byte (with values 00, 01, 10 or 11) would describe the color for a pixel, so one byte stored four pixels and ANTIC rendered them from a shift register. For character modes, the data in memory simply described the index into the character set, and ANTIC stored the individual pixel data for the character in a shift register.
The display list could specify for each line the address in memory from which to fetch the data (with some trivial memory alignment restrictions), but if this was not specified then ANTIC would fetch the data from the next address in memory from where it finished reading the previous line.
Short sections of machine language code could be executed during the horizontal blanking interval, and this was typically done to change the values in the color registers, horizontal sprite positions and so forth thus giving the appearance of more colors or more flexible sprites than the hardware provided ab initio. This machine code had to be very short as there were not many clock cyles available during each horizontal blank. These routines were known as DLIs (display list interrupts) but were simply off limits to Atari BASIC as it would have been far too slow to perform even the simplest tasks. Strictly these should be called "DLI routines" but were usually just called "DLIs".
During the vertical blanking interval, a much longer interval, another interrupt was generated and the Operating System hooked into this to perform some housekeeping tasks. Again, this was not available to Atari BASIC directly, although with some manipulation and severe restrictions (because BASIC was not designed to be re-entrant) it was possible for a VBI (vertical blank interrupt) routine to call a BASIC routine.
Operating system support
The operating System provided several standard "Graphics modes" by which it set up a display list automatically, and allocated memory, at the top end of free memory. These provided a range of graphics modes including text modes, graphics modes and mixed text-and-graphics modes. It was only these predefined modes that were available to Atari BASIC.
The lack of an OS routine for a general-purpose memory move routine perhaps exhibited in artifacts of the graphcs design. For example, once a graphics channel had been opened, it was not possible (or at least easy) to move it down in memory so that other memory could be reserved above it, which meant that generally a program would allocate "more than enough" memory above the high memory pointer (HIMEM) then set the graphcis mode (with the <code>GRAPHICS statement) to have the operating system allocate memory for the display below the new high water mark.
Most of ANTIC's registers were write-only, their values could not be read (or rather they could be, but were meaningless or returned different values from those written as they were multiplexed). The Operating System kept copies of the values written in "shadow registers" in pages 0 and 2 of memory (page 1 was the hardware stack on 6502 processors), thus allowing programs to read the values. The values written here were rewritten to the ANTIC registers during the vertical blank interrupt.
The operating system provided access to the graphics in two ways: by allowing direct reads and writes (in Atari BASIC, through the
POKE commands to the memory being used to hold the graphics (by making available the address of the start of that memory in a well-known location), and to the shadow registers, and also by providing a CIO device, "S:", through which CIO commands could be issued. The S: device supported the general-purpose
XIO command used to implement
FILL (unfortunately the last was not exposed to Atari BASIC, and was also rather tricky to get right at the best of times, as it used a rather primitive scanline fill that filled lines left-to-right, bottom-to-top but stopped as soon as a boundary was met, rather than providing a full flood fill).
In a way there was some confusion and overlap here in the design. For example, the CIO
POINT commands could be considered analogous to reading and writing the position of the cursor, but instead they had a separate interface through well-defined memory locations. Thus the exposure of the graphics API to BASIC and other languages was perhaps not as orthogonal and device-independent as it could have been.
Atari BASIC support
Atari BASIC supported graphics using the statements
Because of the support provided by the operating system, Atari BASIC implemented most of its graphics statements as simple calls to those routines or just set the memory registers for the cursor position and so on. In many cases it simply left programmers to use
POKE statements. It could be argued that some statements such as
SETCOLOR<code> were not only redundant but confusing, since they simply set one color value in a shadow register and could be as easily, and more quickly, done with a <code>POKE<code> command; the ROM space used by these routines could perhaps have been better used to implement other things.
The lack of a <code>FILL command is a notable omission considering that the routine, however primitive, was available in the operating system. It could be achieved with the general-purpose
XIO command, but was rather fiddly:
REM The co-ordinates of the corners of the fill quadrilateral have to be set up REM before calling XIO, using POKE into the IOCB. This is quite a trick because REM it's not easy to find out where the IOCB is. Anyway, then we do: XIO 18,#6,12,0,"S:" REM XIO # = Extended IO. REM 18 = Fill (17=Drawto). REM #6 = On Channel 6, mapped to the graphics screen device. REM 12 = Read/write. REM 0 = Redundant (unused). REM "S:" = Logical device, used only for OPEN and some disk REM commands with a target such as for a RENAME REM Redundant here but used by convention
There was no BASIC support for sprites, although they were not particularly difficult to program in BASIC using
POKE, but this could not be done particularly fast in, say, games. Similarly, setting up a custom display list could be done, but was rather difficult, and was far more easily and effectively done in machine language. Sprite data could be defined using a
DATA statement, and then
POKEd into memory. However, scrolling a sprite vertically, for example, was not quick in BASIC as there was no "block memory move" statement and required a slow
FOR loop of
In comparison to the BASICs of some competing machines at the time, Atari BASIC had good built-in support of sound, (
SOUND statement), graphics (
GRAPHICS, SETCOLOR, COLOR, PLOT and
DRAWTO) and peripheral units like joysticks (
STICK, STRIG) and paddles (
PADDLE, PTRIG). Other home computer users were often left with cryptic
POKEs for such programming.
That being said, the parameters for many of these commands were cryptic, and essentially little better than machine code.
SOUND took four numeric parameters for pitch, tone, volume and channel (the Atari 8-bits had 4-channel sound); the
GRAPHICS statement took three to handle the numerous graphics modes,
SETCOLOR<code> and <code>COLOR each took a number of parameters with different meanings depending on the graphics mode and often which did not match between the two, and so forth. It may be an example of Conway's law: clever designers made excellent hardware, by and large following a common model (memory-mapped register addressing for ANTIC, GTIA and Pokey, for example), but the lack of the teams' interaction made them work in curiously different ways. One may wonder why it would be thought so important to include two key words for examining the state of paddles – something that could be done easily with a single
PEEK and indeed in every respect more efficiently than a
PADDLE statement – yet not have a
FILL command that was already coded in the OS and would have been uniquely advanced for the BASICs of the time.
Similarly, advanced aspects of the hardware such as sprites were completely out of bounds for BASIC programmers, and the lack of access to timers made sound programming difficult, particularly because North American machines ran on different clock speeds from the rest of the world (basically because they were tied to the speed of the television system).
Atari BASIC was slower than other BASICs, sometimes by a surprising amount given the higher speeds of the underlying hardware. Most of these problems stemmed from two particularly poorly implemented bits of code.
One was a side-effect of the way that Atari BASIC recalculated line numbers as the program was being run. This meant that a
GOTO had to run a small amount of additional code in order to find the line to jump to. This would normally be a minor issue, but the same code was also used to implement the "reverse jump" at the end of
NEXT loops, dramatically lowering overall performance of these very common structures. It is fairly obvious that a line number can be stored in a 16-bit integer, but presumably the designers chose to store it that way for other design reasons.
Atari BASIC didn't support integer variables, so all numeric operations were in floating point. Atari BASIC relied on the Atari OS's built-in floating point routines (BCD notation), which were slow. Most of this was due to a particularly poor implementation of the multiply code that was used throughout the math libraries. This was arguably not a problem of the language itself but the underlying OS, but it added to the general poor performance.
The MOS 6502 processor had a special mode for dealing with BCD (the
CLD instructions to treat each 4 bits of a byte as a BCD digit), and perhaps that was particularly attractive to the designers for implementing floating point as BCD. The now almost universal IEEE 754 standard of representation of floating point numbers was still at the design stage when the Atari 8 bit family and its contemporaries first came to market, so the design of an FP implementation was very much up to the OS or BASIC designer.
Several commercial and shareware BASICs were available on the platform that addressed some or all of these issues, resulting in performance that was 3 to 5 times faster than the Atari version. Using these BASICs, the Atari was one of the fastest home computers of its era.
Despite its small footprint (8 kilobytes), Atari BASIC had some features that gave it some powers of more-advanced versions of BASIC.
Atari BASIC had no implementation of user functions. However, programmers could simulate user functions because of the way the GOSUB command could reference a variable. For example, a programmer could start a subroutine at line 10000 and have the program initialize a variable with that number, e.g.
LET TEST = 10000. The calling code could then initialize some mutually understood variables and use the statement
GOSUB TEST to invoke the subroutine. The subroutine starting at line
TEST would then do its operation on the predetermined variables and put return results into variables available after
Atari BASIC could import lines of code and merge them into a single program as long as the line numbers didn't conflict. The code to be merged was written to a medium in textual form using the
LIST command, and could be put back into the program with the
ENTER command. So the stream of text on the medium was, from the BASIC interpreter's point of view, no different from had it been typed at the keyboard or any other input device.
By carefully using blocks of line numbers that did not overlap, programmers could build libraries of subroutines (simulating functions as above) and merge them into new programs as needed.
Embedded machine language
Unlike, for example, the BBC Microcomputer, Atari BASIC did not have a built-in assembly language. Machine code was generally stored as bytes in strings. Machine code functions were invoked via the
USR statement, which worked in much the same way as
GOSUB, but with fewer guarantees. (Cross your fingers.)
String variables could hold any of the 256 characters available in the ATASCII character set and thus each byte of memory reserved for a string variable could hold any number from 0 to 255. Short relocatable 6502 machine language routines could be converted to ATASCII characters and stored as a string variable. The machine language routine could be called as a function with the
USR<code> command specifying the address of the string variable as the location in memory to execute. For example, if the machine language code was stored in a string named <code>ROUTINE$ it could be called and parameters passed to it with
ANSWER=USR(ADR(ROUTINE$),VAR1,VAR2). Parameters were pushed onto the hardware stack as 16-bit integers. Similarly, the return value was expected to be a 16-bit integer returned in the machine registers X and A (since, obviously, the return address of the routine must needs be at the top of the stack).
These routines had to be relocatable (no absolute references such as
JMP, only branch-type jumps) because the strings could be moved around in memory. For this reason Page 6 (060016–06FF16), a page of memory not used by ATARI BASIC or ATARI OS, was very popular for storing small routines.
On the 6502, relocation is not trivial. These days we expect programs to sit pretty much anywhere in memory; the loader and processor collaborate to make that happen. But microprocessors of that era did not do that. The 6502 was especially hindered by having very few indirection instructions, and those it had were asymmetric: the
Y registers indirect in different directions. This leads either to rather clumsy code that is forever moving stuff between registers, or clever but obtuse code that keeps them where they need to be even if it would seem more obvious to stick something else there. The 6502 instruction set is small enough that, over a short time, programmers can model the entire processor in their heads, even down to knowing how many cycles each instruction takes, and then start making clever tricks.
As well as using machine code for advanced functions, fairly trivial
USR routines were sometimes used simply to get access to OS functions that had not been provided through Atari BASIC: for example block serialization to and from devices (BASIC only let it be done byte by byte, with
PUT, which had enormous unnecessary overhead for streaming a block of I/O), or for reading and writing memory (the
POKE commands were also unnecessarily slow because of the numeric problems described above.
Running without Atari BASIC
On the XL/XE models, Atari BASIC could be disabled by holding down the OPTION key while booting the computer. The XEGS would disable BASIC if powered without the keyboard attached.
If another cartridge were inserted it may also disable Atari BASIC, if they used the same address space.
- BASIC A+ – An extended BASIC for the Atari, from Optimized Systems Software (OSS)
- Turbo-Basic XL - Freeware BASIC compatible with ATARI BASIC, also available with a compiler for greater speed and extra commands.
- The ATARI BASIC Reference Manual, Atari Inc, 1980, archived from the original on May 1, 2005, http://web.archive.org/web/20050501034651/http://www.strotmann.de/twiki/bin/view/Infothek/AtBasicReferenceMa
- Wilkinson, Bill (1983), The Atari BASIC Source Book, Optimized Systems Software,Inc., ISBN 0-942386-15-9, http://users.telenet.be/kim1-6502/6502/absb.html, retrieved 2009-04-04
- Wilkinson, Bill (1982), Inside Atari DOS, Optimized Systems Software,Inc., ISBN 0-942386-02-7, http://www.atariarchives.org/iad/introduction.php, retrieved 2009-04-04
- "10: ATARI BASIC", De Re Atari, AtariArchives.org, http://www.atariarchives.org/dere/chapt10.php, retrieved 2009-04-04 – A detailed description of the dialect and interpreter
- ↑ Wilkinson, Bill (1983), The Atari BASIC Source Book, Compute! Books, ISBN 0-942386-15-9
- ↑ See Von Neumann Architecture
- ↑ The main differences were whether strings were allowed to grow and shrink in size once memory had been allocated for them, and whether the size of the string was constant from the outset (e.g. being padded with some special character meaning "end of string" or whether the size was stored independently. Both approaches have advantages and disadvantages, depending on how one is expecting them to be used.
- ↑ In the Wilkinson's BASIC book.
- ↑ Some programs such as PrettyPrint exist which do so by sending the output as graphics to the printer, so that seven lines of 8-bit-high text are printed as eight passes of the 7-bit-high print head.
- ↑ although Wilkinson tends to the parse tree as a set of tables which is really an implentation detail
- ↑ Although 0 is actually explicitly disallowed here by BASIC assuming it to be a coding error, isn't it?
- ↑ Because the literal line numbers are stored as FP, deliberate obfuscation can happen by changing them to values other than natural numbers
- ↑ The number is derived from signal noise on a hardware chip, POKEY, and so is not a pseudorandom number.
- Atari BASIC, The Good, the Bad, and the Ugly
- Albrecht, Bob; Finkel, LeRoy; Brown, Jerald R. (1979), http://www.atariarchives.org/basic/ Atari Basic - A Self-Teaching Guide
- Albrecht, Bob; Finkel, LeRoy; Brown, Jerald R. (1985), http://www.atariarchives.org/basicxl/ Atari Basic - XL Edition
- XFormer, a free Atari Emulator which needs no ROMs or other items