Atari 800 Basic Reference Manual

USR
36 Function Library
Although this is not a function, it is included in this section because it is closely
associated
with the PEEK function. This POKE command inserts data into the
memory location or modifies data already stored there . In the above format,
aexp1 is the decimal address of the location to be poked and aexp2 is the data to
be poked. Note that this number is a decimal number between 0 and 255. POKE
cannot be used to alter ROM locations. In gaining familiarity with this command
it is advisable to look at the memory location with a PEEK and write down the
contents of the location. Then, if the POKE doesn't work as anticipated, the
original contents can be poked into the location.
The above Direct mode example changes the left screen margin from its default
position of 2 to a new position of 10. In other words, the new margin will be 8
spaces
to the right. To restore the margin to its normal default position, press
ti"ti•341;1J1jj
0
Format:
Example:
USR (aexp1 [, aexp2][, aexp3 .. .])
100 RESULT = USR (ADD1,A *2)
This function returns the results of a machine-language subroutine. The first ex-
pression, aexp1, must be an integer or arithmetic expression that evaluates to an
integer that represents the decimal memory address of the machine language
routine to be performed. The input arguments aexp2, aexp3, etc., are optional.
These should be arithmetic expressions within a decimal range of 0 through
65535. A non-integer value may be used; however, it will be rounded to the
nearest integer.
These values will be converted from BASIC's Binary Coded Decimal (BCD)
floating point number format to a two-byte binary number, then pushed onto
the hardware stack, composed of a group of RAM memory locations under
direct control of the 6502 microprocessor chip. Figure 6-1 illustrates the struc-
ture of the hardware stack.
(Number of arguments on the stack-may be 0)
(High byte of argument X)
(Low byte of argument X)
(High byte of argument Y)
(Low byte of argument Y)
(High byte of argument Z)
(Low byte of argument Z)
(Low byte of return address)
(High byte of return address)
Figure 6-1. Hardware Stack Definition
Note: X is the argument following the address of the routine, Y is the
second, Z is the third, etc. There are N pairs of bytes.
See Section 11 for a description of the USR function in machine language pro-
gramming. Appendix D defines the bytes in RAM available for machine
language programming.

Atari 800 Specifications

General

Brand	Atari
Model	800
Category	Desktop
Language	English