Commodore Plus 4 - Page 269

The 6502 Microprocessor 257
. 2018
. 2019
. 201B
EE 15 FF IM3 $FF15
CA
DO E7
00
DEX
BITE 12002
BRK
Increment the background color to
white.
Decrement the counter.
If not zero, do it again.
Stop processing.
The values in $2003 and $200F may be changed to allow experimentation with
the raster line values.
CPX—Compare Memory with. X Register
The value currently in the X register is compared with the specified operand and
the appropriate flags are set.
Operation: X - M Flags Affected: N, Z, C
Addressing Mode Syntax Opcode Bytes Cycles
Immediate CPX#$hh EO 2 2
Zero page CPX $hh E4 2 3
Absolute CPX $hhhh EC 3 4
The value in memory is subtracted from the value in the X register, 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 N Flag Z Flag C Flag
X < memory 1 0 0
X = memory Oil
X > memory 0 0 1
Example: This program copies the contents of $2100-21 OF to $2110-$211F. The X register
is initialized to $10 and incremented until it reaches $1F. The CPX instruction is
used to determine this.
. 2000 A2 10 LDX #$10 Initialize X to $10.
. 2002 BD FO 20 LDA $20F0,X Get the value in originating location.
. 2005 9D 00 21 STA $2100,X Store the value in destination
location.