ARM INSTRUCTION BASICS

By: Kieffer Gray, Eric Werner, and Yemane Abebe

Topics  Instruction Basics and rules  Types of operands  Condition Codes and flags

Instructions  An ARM instruction is 32 bits long.

 About 4 billion different machine instructions.

 Assembler handles most of the detail.

 Some ARM processors support 16-bit Thumb’ instructions.

 Viewed as a compressed form of a subset of the ARM instruction set.

www.arm.com

Programmer Abilities  • • • • Data processing instruction.

Arithmetic and logical operations on data values in registers.

Only instructions which modify values.

Two operands, single result Ex. Add two values to produce single result.

 Move data around  Control sequence of program

ARM data processing rules  All operands are 32 bits wide and come from registers or are specified as literals in the instruction itself.

 The result, if there is one, is 32 bits wide and is placed in a register.

 Each of the operand registers and the result registers are independently specified in the instruction. ‘3-address’ format used.

Simple Register Operands  General format: Op-code operand1, operand2, operand3 ; comment  Arithmetic  Bit-wise  Register movement  Comparison http://www.cs.uregina.ca/Links/class-info/301/ARM-arithmetic/lecture.html

Arithmetic

Bit-wise

Register movement

Comparison

Topics  Instruction Basics and rules  Types of operands  Condition Codes and flags

Immediate Operands  An immediate value can replace the second source operand   Value is a constant preceded by ‘#’ Hexadecimal may be used by also adding ‘&’  ADD r3, r3, #1 ; r3 := r3 + 1 ADD r3, r3, #&f ; r3 := r3 + 15

Immediate Operands 

Shifted Register Operands 

Shifted Register Operands  Shift operations:      LSL: logical shift left 0 - 31 places; pad with 0s LSR: logical shift right 0 - 31 places; pad with 0s ASR: arithmetic shift right 0 - 31 places: pad with 0s if operand was positive, 1s in negative ROR: rotate right by 0 - 32 places; pad with bits that fall off RRX: rotate right extended by 1 place; shifts one place to the right and fills vacated bit with old C flag

Shifted Register Operands

Topics  Instruction Basics and rules  Types of operands  Condition Codes and flags

Setting The Condition Codes



The Flags

 N: Negative  Z: Zero  C: Carry  V: Overflow

What Do They Mean?

 N: The N flag is set by an instruction if the result is negative  Z: The Z flag is set if the result of the instruction is 0  C: The C flag is set if the result of an instruction overflows the 32-bit result register  V: The same as the C flag but for signed operations

How Are the Flags Set?

 Comparison operations always set the condition codes  No option available when using comparison operations  Data processing instructions can set codes if explicitly done

How are the Flags Explicitly Raised?

  Flags are explicitly raised by adding an ‘s’ to the op code (data processing instructions) ‘s’ stands for “set condition codes”  Gives programmer more control ADDS r2, r2 ,r0 ; set condition codes ADD r2,r2,r0; don’t set condition codes

Multiplication  Multiplication instruction has important differences from other arithmetic instructions  Negative and Zero flags are more important if ‘s’ is appended to opcode   Immediate second operand not supported Result register can’t be the same as the sources register

Comparison of MUL and ADD 

ADDS r2, r2, #5; r2:=r2+5

 MULS r4, r3, r2; r4:= (r3 x r2)

Use of the Condition Codes  Most important use of condition codes is for branching  Controls program flow

Example

Table of Branch Conditions