ARM Shifts, Multiplies & Divide??

MVN

Pseudo Instructions Pseudo Intruction: Supported by assembler, not be hardware

Pseudo Instructions Pseudo Intruction: Supported by assembler, not be hardware

Pseudo Instructions Pseudo Intruction: Supported by assembler, not be hardware Replaced with other instruction(s)

Rotate & Shifts

Rotate Rotate : Move bits, overflow wraps around – Rotate Right 4:

Rotate Rotate Right: RORrd, rs, #___ rd  rs Shifted right # bits ROR rd, rs, rm rm

Rotate Rotate Left: Just rotate right by (32 – x bits) Left 12 bits = Right 20 bits

Logical Shift Logical Shift: Move bits, fill gaps with 0, drop overflow – Left Shift 4: – Right Shift 4:

LSL, LSR Logical Shift Left/Right LSLrd, rs, #___ rd  rs Shifted left # bits LSRrd, rs, #___ right LSLrd, rs, rm rd  rs Shifted left rm bits LSRrd, rs, rm right

Arithmetic Shift Arithmetic Shift Right: Move bits right, fill gaps with most significant bit, drop overflow – ASR 4 on Positive: – ASR 4 on Negative:

Arithmetic Shift Arithmetic Shift Right ASRrd, rs, #___ rd  rs Shifted right # bits ASRrd, rs, rm rm

Shifting & Math Shifting right N bits divides by 2 N

Shifting & Math Shifting left N bits multiplies by 2 N

Shift Architecture Shifter processes second operand to ALU

Shift Architecture Can specify a shift to apply to operand2 in other instructions: mov op1, op2, SHIFT_TYPE SHIFT_AMT

Shift Architecture Shifts are Pseudo Instructions implemented with MOV:

Shift Architecture Most data instructions support shifts operation dest, op1, op2, SHIFT_TYPE SHIFT_AMT

Multiply

Multiplier hardware separate from ALU

Basic Multiply MUL : Multiply MULrd, rs, rm rd  rs * rm No shifts or constants for rm!!!

Multiply & Accumulate MLA : Multiply with accumulate MLArd, rs, rm, rn rd  (rs * rm) + rn No shifts or constants Add results of multiply to r11

Divide

Multiply Divide??

Multiply Beefier ARM processors support – SDIV : Signed division SDIVrd, rs, rm rd  rs / rm – UDIV : Unsigned division UDIVrd, rs, rm rd  rs / rm

Multiply Divide?? – Powers of 2 : Shift! – Write a function to divide via subtraction – Do insane bit twiddling Like This Like This

High Level Languages

Shifts High level languages often provide bit shift Right shift may be arithmetic or logical!