Instruction Syntax

Mnemonic	Format	Flags
subf	rD,rA,rB	OE = 0, Rc = 0
subf.	rD,rA,rB	OE = 0, Rc = 1
subfo	rD,rA,rB	OE = 1, Rc = 0
subfo.	rD,rA,rB	OE = 1, Rc = 1

Instruction Encoding

Field	Bits	Description
Primary Opcode	0-5	011111 (0x1F)
rD	6-10	Destination register
rA	11-15	Source register A
rB	16-20	Source register B
OE	21	Overflow Exception
XO	22-29	40 (Extended opcode)
Rc	30-31	Record Condition Register

Operation

rD ← (rB) - (rA)

The value in rA is subtracted from the value in rB, and the result is placed into rD.

Note: The subf instruction performs subtraction. If OE = 1, overflow exceptions are enabled. If Rc = 1, the condition register is updated.

Affected Registers

Condition Register (CR0 field)

(if Rc = 1)

• LT (Less Than)
• GT (Greater Than)
• EQ (Equal)
• SO (Summary Overflow)

Note: CR0 field may not reflect the infinitely precise result if overflow occurs (see XER below).

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 Subtraction

subf r3, r1, r2     # r3 = r2 - r1
subf. r3, r1, r2    # Same as above, but also sets condition register

Simple Arithmetic

# Subtract two values and store result
lwz r4, 0(r10)     # Load first value
lwz r5, 4(r10)     # Load second value  
subf r6, r4, r5     # Subtract values

Overflow Detection

subfo r3, r1, r2    # Subtract with overflow exception enabled
# If overflow occurs, an exception is raised

Related Instructions

subfc, subfe, subfic, subfme, subfze