Commodore Plus 4 - Page 268

Operation: A - M
Addressing Mode
Immediate
Zero page
Zero page, X
Absolute
Absolute, X
Absolute, Y
(Indirect, X)
(Indirect), Y
Syntax
CMP #$hh
CMP $hh
CMP $hh,X
CMP $hhhh
CMP $hhhh,X
CMP $hhhh,Y
CMP ($hh,X)
CMP ($hh),Y
* Add 1 when a page boundary is crossed.
F/a^ Affected: N, Z, C
Opcode
C9
C5
D5
CD
DD
D9
Cl
Dl
.Bytes
2
2
2
3
3
3
2
2
Cycles
2
3
4
4
4*
4*
6
5*
The value in memory is subtracted from the value in the accumulator, but
neither value is altered. The following table shows the effect on each of the flags.
The N flag should be used when a two's complement compare is required (e.g.,
$FF is minus 1 and less than $01). The C flag should be used when an unsigned
compare is required (e.g., $FF is 255 and greater than $01).
Condition
NFlag ZFlag C Flag
A < memory
A = memory
A > memory
1
0
0
0
1
0
0
1
1
Example: In this program, the CMP instruction is used to compare a value in the accumula
tor with the current raster line (register $FF1D). When a match is found, the
background color is changed. The routine is executed 256 times by using the X
register to count.
2000 A2 00
S002 A9 30
LDX #$00
LDA #$30
. 2004 CD ID FP CMP $FF1D
. 2007 DO FB BETE $2004
. 2009 A9 70 LDA #$70
. 200B 8D 15 FF STA $FF15
. 200E A9 50 LDA #$50
. 2010 CD ID FF CMP $FF1D
. 2013 DO FB BNE $2010
Start the counter at zero.
Set the accumulator to the upper
raster value desired.
Compare with current raster line.
Go back if it is not equal.
Get the value for the color black.
Store it in the background color
register.
Set the accumulator to the lower
raster value desired.
Compare with current raster line.
Go back if it is not equal.