One page of ARM

Very brief summary of ARM instructions. Our Resources page has links to more complete tutorials and readings about ARM assembly.

Registers

There are 16 registers r0 to r15. Most are general-use, a few are dedicated for particular purpose. Registers designed as "scratch" (r0-3, r12) are callee-owned, the remainder are caller-owner.

Register Purpose
r0 function param 1, function return, scratch
r1 param 2, scratch
r2 param 3, scratch
r3 param 4, scratch
r4 - r10  
r11 Frame pointer (fp)
r12 Intraprocedural scratch (ip)
r13 Stack pointer (sp)
r14 Link register (lr)
r15 Program counter (pc)

Common instructions

Opcode Instruction Syntax Notes
      Data Processing instructions
ADD Add ADD dest, op1, op2  
ADC Add with Carry ADC dest, op1, op2  
SUB Subtract SUB dest, op1, op2  
SBC Subtract with Carry SBC dest, op1, op2  
RSB Reverse Subtract RSB dest, op1, op2  
RSC Reverse Subtract with Carry RSC dest, op1, op2  
AND Bitwise And AND dest, op1, op2  
EOR Bitwise Exclusive Or EOR dest, op1, op2  
BIC Bitwise Clear BIC dest, op1, op2  
ORR Bitwise Or ORR dest, op1, op2  
CMP Compare CMP op1, op2 op1 - op2 (next 4 insns set flags, discard result)
CMN Compare Negated CMN op1, op2 op1 + op2
TST Test TST op1, op2 op1 & op2
TEQ Test Equals TEQ op1, op2 op1 ^ op2
MOV Move MOV dest, op2  
MVN Move Negated MVN dest, op2 bitwise inverse
       
LDR Load Register LDR dest, [src]  
STR Store Register STR src, [dest] Minor anomaly: src register listed first
B Branch B target  
BL Branch and link BL target Function call
BX Branch exchange BX lr Function return
  • Op2 accepts an immediate value. In the data processing instructions, dest and op1 can only refer to a register, but op2 accepts an immediate value encodable in a 8-bit value with 4-bit rotate. (Here is a neat exploration of what can be encoded under those constraints: https://alisdair.mcdiarmid.org/arm-immediate-value-encoding). For those of you pondering the need for both SUB and RSB, consider the impact of the asymmetry between op1 and op2 on the non-commutative subtract operation. 
    SUB r1, r1, #3
RSB r1, r1, #3
  • Use of barrel shifter on op2. Another distinction for op2 of the data processing instructions is that the barrel shifter can be used to first shift or rotate this operand's value. For example, in the instruction below, r3 is left shifted 5 positions before being added to r2. 
    ADD r1, r2, r3, LSL #5

    The available barrel shifter operations for op2 are below. Here is nice explanation of the effect of each http://www.davespace.co.uk/arm/introduction-to-arm/barrel-shifter.html.

    LSL Logical Shift Left op2, LSL #Imm  
    LSR Logical Shift Right op2, LSR #Imm  
    ASR Arithmetic Shift Right op2, ASR #Imm  
    ROR Rotate Right op2, ROR #Imm  
    RRX Rotate Right and Extend op2, RRX #Imm  

    ARM does not have separate shift/rotate instructions; LSL LSR ASR are implemented as use of barrel shifter on op2.

    To read more about the ARM Flexible second operand, see https://developer.arm.com/documentation/dui0552/a/the-cortex-m3-instruction-set/about-the-instruction-descriptions/flexible-second-operand

Conditional execution

  • Condition codes. The ARM has four condition codes Z N C and V that each store a 1-bit state. The codes are set as a side effect of the Compare and Test instructions and can be optionally set by any data processing instruction by adding the S suffix (XOR -> XORS).

    Z Zero
    N Negative
    V Overflow
    C Carry

  • Conditional execution. Adding a condition suffix to instruction opcode, e.g. (ADD -> ADDEQ) causes the instruction to execute only if the specified condition is met, otherwise the instruction is skipped. For an instruction specified with no suffix, AL is assumed.

    Condition Opcode Execute if:
    EQ Equal Z
    NE Not equal !Z
    CS/HS Carry set / unsigned higher or same C
    CC/LO Carry clear / unsigned lower !C
    MI Minus / negative N
    PL Plus / positive or zero !N
    VS Overflow V
    VC No overflow !V
    HI Unsigned higher C and !Z
    LS Unsigned lower or same !C or Z
    GE Signed greater than or equal N == V
    LT Signed less than N != V
    GT Signed greater than !Z and (N == V)
    LE Signed less than or equal Z or (N != V)
    AL Always (unconditional) always

More

This page is just the highlights – there is much more! Refer to the ARM manual for information on:

  • pre/post-index address modes
  • LDM and STM, multiple load/store
  • the CPSR special register and instructions msr and mrs