Atari 800 User Manual

Atari BASIC Reference Guide 183
The address indicated is that of the machine language
subroutine to be branched to . Function arguments between 0
and 65535 can be optionally included with the USR command as
indicated in the Configuration.
Beginning with the last argument, each argument is evaluated
and converted to a 2-byte hexadecimal integer. This integer is
placed on the hardware stack, and a count of the USR arguments
is also pushed on the stack. The hardware stack configuration is
depicted in Illustration 5-2.
Returning To
BASIC
When BASIC executes a USR function, the BASIC program's
current location is pushed onto the hardware stack (see
Illustration 5-2). The machine language program can return to
BASIC by executing the assembly language RTS instruction . RTS
will pull the return location within the BASIC program from the
hardware stack.
However , before RTS can be used to pull the return location off
the stack, all data on the stack related to function arguments
must have
been pulled off the stack. This includes both the
arguments
themselves as well as the argument count. Even if
there are not arguments, the machine language program must
pull the argument count off the stack before returning to the
BASIC program .
VAL
The VAL function converts its string argument to a numeric
value. The first character of the string argument must be a
numeric character. Otherwise , an error will occur. The numeric
characters in the string argument will be converted to their
numeric equivalents until a non-numeric string character is
encountered .
CONFIGURATION
VAL (a$)

Atari 800 Specifications

General

Processor	MOS Technology 6502
Processor Speed	1.79 MHz
ROM	10 KB
Graphics	ANTIC and GTIA chips
Release Year	1979
Display Resolution	up to 320 x 192
Colors	128 colors
Sound	4 channels
Storage	Cartridge, cassette, floppy disk
Ports	Cartridge, serial, monitor
Operating System	Atari OS