neg
Negate - 7C 00 00 D0
neg
Instruction Syntax
Mnemonic | Format | Flags |
neg | rD,rA | OE = 0, Rc = 0 |
neg. | rD,rA | OE = 0, Rc = 1 |
nego | rD,rA | OE = 1, Rc = 0 |
nego. | rD,rA | OE = 1, Rc = 1 |
Instruction Encoding
0
1
1
1
1
1
D
D
D
D
D
A
A
A
A
A
0
0
0
0
0
OE
0
0
0
0
0
0
0
0
0
Rc
Field | Bits | Description |
Primary Opcode | 0-5 | 011111 (0x1F) |
rD | 6-10 | Destination register |
rA | 11-15 | Source register |
Reserved | 16-20 | 00000 |
OE | 21 | Overflow Exception |
XO | 22-29 | 208 (Extended opcode) |
Rc | 30-31 | Record Condition Register |
Operation
rD ← ¬(rA) + 1
The two's complement of (rA) is placed into rD.
Note: The neg instruction computes the arithmetic negation by taking the two's complement of the source value.
Affected Registers
Condition Register (CR0 field)
(if Rc = 1)
- LT (Less Than)
- GT (Greater Than)
- EQ (Equal)
- SO (Summary Overflow)
XER (Exception Register)
(if OE = 1)
- SO (Summary Overflow)
- OV (Overflow)
For more information on condition codes see Section 2.1.3, "Condition Register," and Section 2.1.5, "XER Register," in the PowerPC Microprocessor Family: The Programming Environments manual.
Examples
Basic Negation
neg r3, r1 # r3 = -r1 neg. r3, r1 # Same as above, but also sets condition register
Absolute Value
# Compute absolute value of r1 cmpwi r1, 0 # Compare r1 with 0 bge positive # If r1 >= 0, skip negation neg r1, r1 # Negate r1 if it was negative positive:
Overflow Detection
nego r3, r1 # Negate with overflow exception enabled # If overflow occurs (e.g., negating -0x80000000), an exception is raised