Instruction Syntax

Mnemonic	Format	Flags
fneg	frD,frB	Rc = 0
fneg.	frD,frB	Rc = 1

Instruction Encoding

Field	Bits	Description
Primary Opcode	0-5	111111 (0x3F)
frD	6-10	Destination floating-point register
Reserved	11-15	00000
frB	16-20	Source floating-point register B
Reserved	21	0
XO	22-30	000101000 (40)
Rc	31	Record Condition Register

Operation

frD ← -frB

The negation of the contents of floating-point register frB is placed into floating-point register frD. The operation is performed by inverting the sign bit of frB.

Note: This instruction does not affect the floating-point status and control register (FPSCR) exception bits, but may affect the FPSCR condition code bits if Rc = 1. The operation is implemented by simply flipping the sign bit.

Affected Registers

Floating-Point Status and Control Register (FPSCR)

(if Rc = 1)

• FPRF (Floating-Point Result Flags) - updated to reflect the result

For more information on floating-point status see Section 2.1.4, "Floating-Point Status and Control Register (FPSCR)," in the PowerPC Microprocessor Family: The Programming Environments manual.

Examples

Basic Negation

lfd f1, input_value(r0)    # Load floating-point value
fneg f2, f1                # f2 = -f1 (negate)
stfd f2, negated_result(r0) # Store negated value

Negation with Condition Recording

lfd f3, input_value(r0)    # Load input value
fneg. f4, f3               # f4 = -f3, set FPSCR condition codes
# FPSCR condition codes now reflect the negated result

Vector Negation

# Negate all components of a 3D vector
lfd f1, vector_x(r0)       # Load vector.x
lfd f2, vector_y(r0)       # Load vector.y
lfd f3, vector_z(r0)       # Load vector.z
fneg f4, f1                # negated.x = -vector.x
fneg f5, f2                # negated.y = -vector.y
fneg f6, f3                # negated.z = -vector.z

Direction Reversal

# Reverse direction vector for physics
lfd f1, velocity_x(r0)     # Load velocity X
lfd f2, velocity_y(r0)     # Load velocity Y
lfd f3, velocity_z(r0)     # Load velocity Z
fneg f1, f1                # Reverse X direction
fneg f2, f2                # Reverse Y direction
fneg f3, f3                # Reverse Z direction
stfd f1, velocity_x(r0)    # Store reversed X
stfd f2, velocity_y(r0)    # Store reversed Y
stfd f3, velocity_z(r0)    # Store reversed Z

Mathematical Operations

# Calculate: result = a - b (using negation and addition)
lfd f1, value_a(r0)        # Load a
lfd f2, value_b(r0)        # Load b
fneg f3, f2                # f3 = -b
fadd f4, f1, f3            # f4 = a + (-b) = a - b
stfd f4, result(r0)        # Store result

Sign Toggle

# Toggle sign based on condition
lfd f1, input_value(r0)    # Load input value
lwz r3, should_negate(r0)  # Load condition flag
cmpwi cr0, r3, 0           # Check if should negate
beq skip_negation          # Skip if flag is 0
fneg f1, f1                # Negate if flag is non-zero
skip_negation:
stfd f1, output_value(r0)  # Store result

Absolute Difference Calculation

# Calculate |a - b| using negation
lfd f1, value_a(r0)        # Load a
lfd f2, value_b(r0)        # Load b
fsub f3, f1, f2            # f3 = a - b
fabs f4, f3                # f4 = |a - b|
fneg f5, f3                # f5 = -(a - b) = b - a
fabs f6, f5                # f6 = |b - a| (same as f4)
# Both f4 and f6 contain |a - b|

Oscillation/Wave Calculations

# Create oscillating value (sine wave approximation)
lfd f1, amplitude(r0)      # Load amplitude
lfd f2, phase_flag(r0)     # Load phase indicator
fcmpo cr0, f2, f0          # Compare with zero
bge positive_phase         # Branch if positive phase
fneg f1, f1                # Negate amplitude for negative phase
positive_phase:
stfd f1, oscillation_value(r0) # Store oscillating value

Color Inversion

# Invert color values (for visual effects)
lfd f1, color_offset(r0)   # Load color offset
lfd f2, red_channel(r0)    # Load red channel
lfd f3, green_channel(r0)  # Load green channel
lfd f4, blue_channel(r0)   # Load blue channel
fneg f2, f2                # Invert red
fneg f3, f3                # Invert green
fneg f4, f4                # Invert blue
fadd f2, f2, f1            # Add offset to red
fadd f3, f3, f1            # Add offset to green
fadd f4, f4, f1            # Add offset to blue

Financial Calculations

# Convert credit to debit (negate amount)
lfd f1, transaction_amount(r0) # Load transaction amount
lwz r3, is_credit(r0)          # Load credit flag
cmpwi cr0, r3, 1               # Check if it's a credit
bne store_amount               # If not credit, use as-is
fneg f1, f1                    # Negate for debit representation
store_amount:
stfd f1, final_amount(r0)      # Store final amount

Physics Force Calculation

# Apply opposite force (Newton's third law)
lfd f1, applied_force_x(r0)    # Load applied force X
lfd f2, applied_force_y(r0)    # Load applied force Y
lfd f3, applied_force_z(r0)    # Load applied force Z
fneg f4, f1                    # reaction_force.x = -applied_force.x
fneg f5, f2                    # reaction_force.y = -applied_force.y
fneg f6, f3                    # reaction_force.z = -applied_force.z
stfd f4, reaction_force_x(r0)  # Store reaction force X
stfd f5, reaction_force_y(r0)  # Store reaction force Y
stfd f6, reaction_force_z(r0)  # Store reaction force Z

Related Instructions

fabs, fnabs, fmr, fsub, fadd