Commodore VIC-20 Programmer's Reference Manual page 217

LDX VARTAB ;Load .X WITH LOW BYTE OF END OF SAVE
LDY VAR
TAB + 1 ; .Y WITH HIGH BYTE
LDA
#<TXTTAB ;LOAD ACCUMLATOR WITH PAGE 0 OFF
SET
JSR SAVE
B-22. Function name: SCNKEY
Purpose: Scan the keyboard
Call address: SFF9F
Com muni cation registers: None
Preparatory routines: None
Error returns; None
Stack requirements: None
Registers affected: .A, .Xt .Y
Description; This routine will scan the VIC keyboard and check
for pressed keys. It is the same routine called by the interrupt
handler. If a key is down, its ASCII value is placed in the keyboard
queue.
How to use;
1) Call this routine
EXAMPLE:
GET JSR SCNKEY ;SCAN KEYBOARD
JSR GETIN ;GET CHARACTER
CMP #0 ;IS IT NULL?
BEQ GET ;YES, . .SCAN AGAIN
JSR CHROUT ;PRINT IT
B-23. Function name: SCREEN
Purpose: Return screen format
Call address: SFFED
Communication registers: X..Y
Preparatory routines: None
Stack requirements: 2
Registers affected: .X, .Y
Description: This routine returns the format of the screen, e.g.,
22 columns in ,X and 23 lines in ,Y. This routine can be used to
determine what machine a program is running on, and has been
implemented on the VIC to help upward compatibility in programs.
Mow to use:
1) Call this routine.
EXAMPLE:
JSR SCREEN
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