DEC DECREMENT
|
Purpose |
subtract 1 from an integer |
|
Format |
opcode dif.mx |
|
Operation |
dif ß dif - 1; |
|
Condition codes |
N ß dif LSS 0; Z ß dif EQL 0; V ß {integer overflow}; C ß {borrow from most significant bit}; |
|
Exceptions |
Integer overflow |
|
Opcodes |
97 DECB Decrement Byte B7 DECW Decrement Word D7 DECL Decrement Long |
|
Description |
One is subtracted from the difference operand and the difference operand is replaced by the result. |
|
Notes |
1. Integer overflow occurs if the largest negative integer is decremented. On overflow, the difference operand is re placed by the largest positive integer. 2. DECx dif is equivalent to SUBx2 $1, dif, but is shorter. |
Example 1:
.text
.word 0
movl $7, r0
decl r0
pushl r0
pushal format
calls $2, .printf
pushl $0
calls $1, .exit
format: .asciz "%d\n"
Example 2
Flags example - the following program demonstrate the different values for flags while performing DEC commands: We can say that N and Z are set as always. V is set if 80..0 is decremented to 7F..F. C is set if 0 is decremented to FF..F.
.text
.word 0
movl $3, r0
decl r0
decl r0
decl r0 # Z = 1
decl r0 # N = 1, C = 1
movb $0x81, r0
decb r0 # N = 1
decb r0 # N = 0, C = 0, V = 1
decb r0 # N = 0, C = 0, V = 0
pushl $0
calls $1, .exit
Example 3
Another flags example
.text
main: .word 0
movb $0, r0
decb r0 # N = 1, C = 1
movb $0xFF, r0
decb r0 # N = 1
movw $0x0000, r0
decw r0 # N = 1, C = 1
movl $0x00000000, r0
decl r0 # N = 1, C = 1
movl $0x80000000, r0
decl r0 # V = 1
movw $0x8000, r0
decw r0 # V = 1
movb $0x80, r0
decb r0 # V = 1
pushl $0
calls $1, .exit