Atari 800 Basic Reference Manual

PROGRAMMING
IN MACHINE
LANGUAGE
7. Each character after REM takes one byte of memory. Remarks are helpful to
people trying to understand a program, but sometimes it is necessary to
remove remark statements to save memory.
8. Subroutines can save memory because one subroutine and several short calls
take less memory than duplicating the code several times. On the other
hand, a subroutine that is only called once takes extra bytes for the GOSUB
and· RETURN statements.
9. Parentheses take one byte each. Extra parentheses are a good idea in some
cases if they make an expression more understandable to the programmer .
However, removing unnecessary parentheses and relying on operator
precedence will same a few bytes.
Machine language is written entirely in binary code. The AT ARI Personal Com-
puter contains a 6502 microprocessor and it is possible to call 6502 machine code
subroutines from BASIC using the USR function . Short routines ma y then be
entered into a program by hand assembly (if necessary).
Before it returns to BASIC, the assembly language routine must do a pull ac-
cumulator (PLA) instruction to remove the number (N) of input arguments off
the stack. If this number is not 0, then all ofthe input arguments must be pop-
ped off the stack also using PLA. (See Figure 6-1).
The subroutine should end by placing the low byte of its result in location 212
(decimal), and then return to BASIC using an RTS (Return from Subroutine) in-
struction. The BASIC interpreter will convert the 2-byte binary number stored
in locations 212 and 213 into an integer between 0 and 65535 in floating-point
format to obtain the value returned by the USR function.
The ADR function ma y be used to pass data that is stored in arrays or strings to a
subroutine in machine language. Use the ADR function to get the address of the
array or string, and then use this address as one of the USR input arguments.
The following program, Hexcode Loader, provides the means of entering hexa-
decimal codes, co nverting each hexadecimal number to decimal, and storing the
decimal number into an array. The array is then executed as an assembly
language subroutine. (An array is used to allocate space in memory for the
routine.)
Advanced Programming Techniques 63

Atari 800 Specifications

General

Brand	Atari
Model	800
Category	Desktop
Language	English