Commodore Plus 4 - Page 255

The 6502 Microprocessor 243
Operation:
Addressing Mode Syntax
Flags Affected: N, Z, C, V
Opcode Bytes Cycles
Example:
Immediate
Zero page
Zero
page, X
Absolute
Absolute, X
Absolute, Y
(Indirect, X)
(Indirect), Y
ADC #$hh
ADC $hh
ADC $hh,X
ADC Shhhh
ADC $hhhh,X
ADC $hhhh,Y
ADC ($hh,X)
ADC ($hh),Y
69
65
75
6D
7D
79
61
71
2
2
2
3
3
3
2
2
2
3
4
4
4*
4*
6
5*
* Add 1 when a page boundary is crossed.
The status of the carry following an addition reflects the result. If a carry was
generated, the carry is set to 1. If no carry was generated, the carry is cleared to 0.
This fact allows multiple precision additions, as shown in the example. 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. If the result exceeds+127 or
-128, the overflow flag is set to 1; otherwise it is cleared to 0.
Frequently it is necessary to have a greater precision in calculations than the 256
possible values for a single byte. This is possible by treating a group of two or
more bytes as a single number. In this example, the two 32-bit numbers stored at
$2100-$2103 and $2104-$2107 (high byte to low byte) are added, and the result is
stored in $2108-$210B.
. 2000 18
. 2001 A2 03
. 2003 BD00 21
. 2006 7D 04 21
. 2009 9D 08 21
. 200C CA
. 200D 10 P4
. 200F 00
CLC
LDX #$03
LDA $2100,X
ADC $2104,X
STA $2108,X
DEX
BPL $2003
BRK
Clear the carry prior to the first
addition.
.X will index through the 4 bytes.
Get a byte of the first number.
Add corresponding byte from the
second number.
Store the result in destination.
Decrement .X to point at the next
byte.
Continue processing until all 4 bytes
are done.
Stop processing. *
Use the M (Memory) command to examine and modify the contents of
$2100-$2107 before executing the program (using G 2000). Then check the