Atari XL User Manual

200
The Afar; XL Handbook
about the XL's microprocessor (the 6502) as it is
very common in personal computers.
You can enter machine code routines from BASIC
using the command USR, giving the start address
in memory of the machine code routine. For
example:
USR (69316)
wil l perform the same function as pressing the
RESET key, and:
USR (69319)
will call the power-on setting up routines which
clear the memory and print the start-up message.
BE CAREFUL using this - it will destroy any
program you have in the XL at the time.
The USR command may also contain one or more
arguments, separated by commas, after the address
of the machine code routine. When the command is
executed, the address of the current BASIC
instruction is placed on the stack, followed by each
of the arguments, followed by the number of
arguments included in the command. The machine
code routine can then use the arguments and
return them to the stack, for use by BASIC.
Machine code subroutines may be stored as string
variables, and another command, AD R, used to
determine the start address in memory of the
string, hence the start address of the machine code
subroutine. The syntax of ADR is:
x = ADR(A$)

Atari XL Specifications

General

Release Year	1983
CPU	MOS Technology 6502C
Graphics	ANTIC, GTIA
Sound	POKEY (4 channels)
Storage	Cassette tape
Ports	Cartridge
Operating System	Atari BASIC
CPU Speed	1.79 MHz
RAM	64 KB