AArch64 Register and Instruction Quick Start
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.
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
- ARM Aarch64 documentation
- ARM Developer Information Centre
- ARM Cortex-A Series Programmer’s Guide for ARMv8-A
- The short guide to the ARMv8 instruction set: ARMv8 Instruction Set Overview ("ARM ISA Overview")
- The long guide to the ARMv8 instruction set: ARM Architecture Reference Manual ARMv8, for ARMv8-A architecture profile ("ARM ARM")
- Procedure Call Standard for the ARM 64-bit Architecture (AArch64)
- ARM Developer Information Centre
- GAS Manual - Using as, The GNU Assembler: https://sourceware.org/binutils/docs/as/