This page contains very basic information on the AArch64 mode of the ARMv8/ARMv9 architecture: the register layout and naming and some basic instructions. For more comprehensive information, see the references listed below.

The aarch64 registers are named:

• r0 through r30 - to refer generally to the registers
  • x0 through x30 - for 64-bit-wide access (same registers, specifically when accessed 64 bits at a time)
  • w0 through w30 - for 32-bit-wide access (same registers, specifically accessed 32 bits (or less) at a time - 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 integer and pointer arguments; additional arguments are on the stack. Return value is in r0 (if an integer or pointer)
• 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.

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 
 Program Counter plus an offset)
 adrp r0,label     // load r0 with the 4K page containing label (this calculates an address from the Program Counter plus an offset, and is often followed by an ADD instruction so that the register points exactly to the label)
 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. The ",0" is not required.
 ldr w0,[r1,0]     // like above but reads 32 bits only - note the use of w0 instead of r0 for the source register name.  The ",0" is not required.
 ldrb w0,[r1,0]    // like above but reads 1 byte (8 bits) only - note the use of w0 for the source register name. The ",0" is not required.
 ldur r0,[r1,0]    // load register r0 from the address pointed to by (r1 + 0) - the mnemonic means "load unscaled register".  The ",0" is not required.
 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 to the register name 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
• For any LDR/STR instruction that is loading or storing less than 64 bits, use the narrow (32-bit) version of the register name. For example, STRB w14b,[x10] would store the lowest byte from register 14 to the address pointed to by register 10.

• ARM Aarch64 documentation (many resources)
  • ARM Developer Information Centre
• ARM Cortex-A Series Programmer’s Guide for ARMv8-A
• Instruction References
  • Armv8 Instruction Set Overview - this is an older document (and not up-to-date with the latest instructions) but it is accessible and compact, and serves as a good quick reference for the base instructions - effectively making a good index for the ARM ARM (below)
  • ARM Architecture Reference Manual ARMv8, for ARMv8-A architecture profile (known as the “ARM ARM”) - this is the “big” document with all the details
• ARM ABI documentation (on GitHub) - see specifically the Procedure Call Standard for the Arm 64-bit Architecture
• GAS Manual - Using as, The GNU Assembler