Commodore Plus 4 - Page 271

2009
SOOB
SOOE
SOOF
SOU
S013
Bl
99
08
CO
90
00
D8
10 SI
10
P6
LDA
STA
INY
CFY
BCC
BRK
($D8),Y
$2110,Y
#♦10
#2009
The 6502 Microprocessor 259
Get the contents of the originating
location.
Store in the destination location.
Increment .Y by 1.
Compare .Y with $10.
If .Y is less than $10, go back.
Stop processing.
Use the M (Memory) command to examine and change the values in
$2100-$211F.
DEC—Decrement Memory "by 1
The value currently in the operand is decremented by 1.
Operation: M - 1 —* M Flags Affected: N, Z
Addressing Mode Syntax Opcode Bytes Cycles
Zero page DEC $hh C6 2 5
Zero page, X DEC $hh,X D6 2 6
Absolute DEC$hhhh CE 3 6
Absolute, X DEC $hhhh,X DE 3 7
The value stored in the operand is decreased by 1. If the result is negative, the N
flag is set to 1; otherwise it is cleared to 0. If the result is zero, the Z flag is set to 1;
otherwise it is cleared to 0. It should be noted that the carry flag is NOT affected.
That is, a decrement from $00 to $FF does NOT result in the carry flag being
cleared.
Example: This program searches the operating system ROM for the last occurrence of an
$EA (the opcode for a NOP). Indirect indexed addressing is used, and the DEC
instruction updates the indirect address.
. 2000 A9 00 LDA #$00 Load .A with a zero.
. 2002 85 D8 STA $D8 Store zero in the low byte of indirect
address.
. 2004 A8 TAY Store zero in .Y.
. 2005 A9 FF LDA #$FF Load .A with $FF.
. 2007 85 D9 STA $D9 Store $FF in the high byte of indirect
address.
. 2009 A9 EA LDA #$EA .A is the value to look for.
. 200B Dl D8 CMP ($D8),Y Examine the current location.