AArch64 Register and Instruction Quick Start

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This page contains very basic information on the AArch64 mode of the ARMv8 architecture: the register layout and naming and the some basic instructions.

Registers

General-Purpose Registers

The aarch64 registers are named:

  • r0 through r30 - to refer generally to the registers
  • x0 through x30 - for 64-bit-wide access (same registers)
  • w0 through w30 - for 32-bit-wide access (same registers - upper 32 bits are either cleared on load or sign-extended (set to the value of the most significant bit of the loaded value)).

Register '31' is one of two registers depending on the instruction context:

  • For instructions dealing with the stack, it is the stack pointer, named rsp
  • For all other instructions, it is a "zero" register, which returns 0 when read and discards data when written - named rzr (xzr, wzr)

Usage during syscall/function call:

  • r0-r7 are used for arguments and return values; additional arguments are on the stack
  • For syscalls, the syscall number is in r8
  • r9-r15 are for temporary values (may get trampled)
  • r16-r18 are used for intra-procedure-call and platform values (avoid)
  • The called routine is expected to preserve r19-r28 *** These registers are generally safe to use in your program.
  • r29 and r30 are used as the frame register and link register (avoid)

See the ARM Procedure Call Reference for details.

Floating-Point and SIMD Registers

Aarch64 also defines a set of large registers for floating-point and single-instruction/multiple-data (SIMD) operations. For details, refer to the ARM documentation.

Instructions

Starter Kit

These instructions are sufficient to complete the SPO600 Assembler Lab; remember to replace the generic register names with ones that specify width (for example, replace "r0" with "x0" or "w0"). 

add r0,r1,r2      // load r0 with r1+r2
add r0,r1,99      // load r0 with r1+99
adr r0,label      // load r0 with the address label (this actually calculates an address from the PC plus an offset)
bl label          // branch (with link) to label - this is a procedure / subroutine / function call
br label          // branch to label - this is a goto
br register       // branch to the address in register
b.eq label        // branch to label if equal
b.ne label        // branch to label if not equal
b.lt label        // branch to label if less
b.gt label        // branch to label if greater
cmp r0,r1         // compare register r0 with register r1. The comparison sets flags in the processor status register which affect conditional branches.
cmp r0,99         // compare the number 99 with register r0. The comparison sets flags in the processor status register which affect conditional branches.
ldr r0,[r1,0]     // load register r0 from the address pointed to by (r1 + (0 * size)) where size is 8 bytes for 64-bit stores, 4 bytes for 32-bit stores
ldr w0,[r1,0]     // like above but reads 32 bits only - note the use of w0 instead of r0 for the source register name
ldrb w0,[r1,0]    // like above but reads 1 byte (8 bits) only - note the use of w0 for the source register name
ldur r0,[r1,0]    // load register r0 from the address pointed to by (r1 + 0) - the mnemonic means "load unscaled register"
mov r0,r1         // move data from r1 to r0
mov r0,99         // load r0 with 99 (only certain immediate values are possible)
ret               // return from subroutine (counterpart to bl)
str r0,[r1,0]     // store register r0 to address pointed to by (r1 + (0 * size)) where size is 8 bytes for 64-bit stores
strb w0,[r1,0]    // like str but writes one byte only - note the use of w0 for the source register name
stur r0,[r1,0]    // store register r0 to the address pointed to by (r1 + 0) - the mnemonic means "store unscaled register"
svc 0             // perform a syscall
msub r0,r1,r2,r3  // load r0 with r3-(r1*r2) (useful for calculating remainders)
madd r0,r1,r2,r3  // load r0 with r3+(r1*r2)
mul r0,r1,r2      // load r0 with r1*r2 (actually an alias - see ARM ARM)
push r0           // push r0 onto the stack
pop r0            // pop r0 off the stack
udiv r0,r1,r2     // unsigned - divide r1 by r2, places quotient into r0 - remainder is not calculated (use msub)

Note the syntax:

  • Register names are not prefixed.
  • Immediate values are not prefixed with a character (they may be prefaced with # if desired).
  • Indirect memory access is indicated by [square brackets].
  • Hexadecimal values are indicated by a 0x prefix.
  • Character values are indicated by quotation marks. Escapes (such as '\n') are permitted.
  • Destinations are given as the first argument (mov r0, r1 moves INTO r0 FROM r1; you can think of this as r0=r1).
  • For the LDR/STR instructions: you can append a character indicating the number of bits (lowest) to be loaded or stored:
    • Q = Quadword = 64 bits
    • D = Double word = 32 bits
    • W = Word = 16 bits
    • B = Byte = 8 bits

Resources

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