Instruction Syntax

Mnemonic	Format	Flags
fsub	frD,frA,frB	Rc = 0
fsub.	frD,frA,frB	Rc = 1

Instruction Encoding

Field	Bits	Description
Primary Opcode	0-5	111111 (0x3F)
frD	6-10	Destination floating-point register
frA	11-15	Source floating-point register A (minuend)
frB	16-20	Source floating-point register B (subtrahend)
Reserved	21-25	00000
XO	26-30	10100 (20)
Rc	31	Record Condition Register

Operation

frD ← (frA) - (frB)

The contents of floating-point register frB are subtracted from the contents of floating-point register frA. The result is placed into floating-point register frD.

Note: Both operands and the result are in double-precision format. The subtraction follows IEEE-754 standard for floating-point arithmetic.

Affected Registers

Condition Register (CR1 field)

(if Rc = 1)

• Reflects floating-point exception summary and status

Floating-Point Status and Control Register (FPSCR)

Affected fields:

• FPRF (Floating-Point Result Flags)
• FX (Floating-Point Exception Summary)
• OX (Overflow Exception)
• UX (Underflow Exception)
• XX (Inexact Exception)
• VXISI (Invalid Operation Exception for ∞ - ∞)
• FR (Fraction Rounded)
• FI (Fraction Inexact)

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 Floating-Point Subtraction

lfd f1, minuend(r0)        # Load minuend (value to subtract from)
lfd f2, subtrahend(r0)     # Load subtrahend (value to subtract)
fsub f3, f1, f2            # f3 = f1 - f2
stfd f3, result(r0)        # Store result

Subtraction with Condition Recording

lfd f1, value_a(r0)        # Load first value
lfd f2, value_b(r0)        # Load second value
fsub. f3, f1, f2           # f3 = f1 - f2, set CR1 with FP status
# CR1 now reflects floating-point exception status

Calculate Difference for Comparison

# Calculate absolute difference between two values
lfd f1, measurement_a(r0)  # Load first measurement
lfd f2, measurement_b(r0)  # Load second measurement
fsub f3, f1, f2            # difference = a - b
fabs f4, f3                # |difference| = |a - b|
stfd f4, abs_difference(r0) # Store absolute difference

Vector Subtraction

# Subtract corresponding components of two 3D vectors
lfd f1, vector1_x(r0)      # Load vector1.x
lfd f2, vector1_y(r0)      # Load vector1.y
lfd f3, vector1_z(r0)      # Load vector1.z
lfd f4, vector2_x(r0)      # Load vector2.x
lfd f5, vector2_y(r0)      # Load vector2.y
lfd f6, vector2_z(r0)      # Load vector2.z
fsub f7, f1, f4            # result.x = vector1.x - vector2.x
fsub f8, f2, f5            # result.y = vector1.y - vector2.y
fsub f9, f3, f6            # result.z = vector1.z - vector2.z

Calculate Change/Delta

# Calculate change in value over time
lfd f1, current_value(r0)  # Load current value
lfd f2, previous_value(r0) # Load previous value
fsub f3, f1, f2            # change = current - previous
stfd f3, delta(r0)         # Store change

Offset Calculation

# Calculate offset from reference point
lfd f1, target_position(r0) # Load target position
lfd f2, reference_point(r0) # Load reference point
fsub f3, f1, f2             # offset = target - reference
stfd f3, offset(r0)         # Store offset

Error Calculation

# Calculate error between expected and actual values
lfd f1, actual_value(r0)   # Load actual measured value
lfd f2, expected_value(r0) # Load expected value
fsub f3, f1, f2            # error = actual - expected
stfd f3, error(r0)         # Store error
fabs f4, f3                # absolute error = |error|
stfd f4, abs_error(r0)     # Store absolute error

Financial Calculations

# Calculate profit/loss
lfd f1, revenue(r0)        # Load revenue
lfd f2, costs(r0)          # Load costs
fsub f3, f1, f2            # profit = revenue - costs
stfd f3, profit(r0)        # Store profit (negative = loss)

Physics/Engineering Calculations

# Calculate displacement: final_position - initial_position
lfd f1, final_position(r0)   # Load final position
lfd f2, initial_position(r0) # Load initial position
fsub f3, f1, f2              # displacement = final - initial
stfd f3, displacement(r0)    # Store displacement

Temperature Difference

# Calculate temperature difference
lfd f1, temp_celsius(r0)   # Load temperature in Celsius
lfd f2, freezing_point(r0) # Load freezing point (0°C)
fsub f3, f1, f2            # degrees above freezing = temp - 0
stfd f3, above_freezing(r0) # Store result

Range Calculation

# Calculate range (max - min)
lfd f1, maximum_value(r0)  # Load maximum value
lfd f2, minimum_value(r0)  # Load minimum value
fsub f3, f1, f2            # range = max - min
stfd f3, range(r0)         # Store range

Complex Number Subtraction

# Subtract two complex numbers: (a + bi) - (c + di)
lfd f1, complex1_real(r0)  # Load real part of first complex
lfd f2, complex1_imag(r0)  # Load imaginary part of first complex
lfd f3, complex2_real(r0)  # Load real part of second complex
lfd f4, complex2_imag(r0)  # Load imaginary part of second complex
fsub f5, f1, f3            # Real part: a - c
fsub f6, f2, f4            # Imaginary part: b - d
# Result: f5 + f6*i

Related Instructions

fadd, fsubs, fabs, fneg, fcmpo