Commodore Plus 4 - Page 273

The 6502 Microprocessor
DEY—Decrement the Y Register by 1
The value currently in the Y register is decremented by 1.
861
Operation: Y- 1 -» Y
Addressing Mode Syntax
Flags Affected: N, Z
Opcode Bytes Cycles
Implied DEY
88
1
Example:
The value stored in the Y register 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.
In this program the contents of $2100-$21FF are copied onto the screen at
$0C00-$0CFF. The Y register is used for indirect indexed addressing, and the
DEY instruction updates the index value.
S000 A9 00
2002 85 D8
LDA #$00
STA ID8
. 2004 85 DA STA $DA
. 2006 A8
• 2007 A9 21
. 2009 85 D9
. 200B A9 OC
. 200D 85 DB
. 200F Bl D8
. 2011 91 DA
. 2013 88
. 2014 DO P9
. 2016 00
TAY
LDA #$21
STA $D9
LDA #$0C
STA $DB
LDA ($D8),Y
STA ($DA),Y
DEY
BITE $200F
BRK
Load .A with zero.
Store zero in the low byte of
origination address.
Store zero in the low byte of
destination address.
Initialize .Y at zero.
Load .A with $21.
Store $21 in the high byte of
origination address.
Load .A with $0C.
Store $0C in the high byte of
destination address.
Get the value of the originating byte.
Store in the destination byte.
Decrement .Y by 1.
If not zero, continue.
Stop processing.
Use the M (Memory) command to put the desired screen code values in
$2100-$21FF (see Appendix E for screen codes).
E0R—Exclusive-OR Memory with. Accumulator
The value currently in the accumulator is logically exclusive-ORed to the speci
fied operand, and the result is placed in the accumulator.