Assembly language changes after RVCTv2.1

The assembly language accepted in RVCT v2.1 assembler and earlier is called pre-UAL ARM and Thumb. The current assembler accepts the UAL and the pre-UAL ARM and Thumb syntax. The assembler accepts the pre-UAL Thumb syntax only if it is preceded by a CODE16 directive, or if the source file is assembled with the --16 command line option.

For the convenience of programmers who are familiar with the ARM and Thumb assembly languages accepted in RVCT v2.1 and earlier, Table 14 highlights the differences between the UAL and pre-UAL ARM assembly language syntax.

Table 14. Changes from earlier ARM assembly language

ChangePre-UAL ARM syntaxPreferred UAL syntax
The default addressing mode for LDM and STM is IA
You can use the PUSH and POP mnemonics for full, descending stack operations in ARM as well as Thumb.
STMFD sp!, {reglist}
LDMFD sp!, {reglist}
PUSH {reglist}
POP {reglist}
You can use the LSL, LSR, ASR, ROR, and RRX instruction mnemonics for instructions with rotations and no other operation, in ARM as well as Thumb.
MOV Rd, Rn, LSL shift
MOV Rd, Rn, LSR shift
MOV Rd, Rn, ASR shift
MOV Rd, Rn, ROR shift
LSL Rd, Rn, shift
LSR Rd, Rn, shift
ASR Rd, Rn, shift
ROR Rd, Rn, shift
RRX Rd, Rn
Use the label form for PC-relative addressing. Do not use the offset form in new code.
LDR Rd, [pc, #offset]
LDR Rd, label
Specify both registers for doubleword memory accesses. You must still obey rules about the register combinations you can use.
LDRD Rd, addr_mode
LDRD Rd, Rd2, addr_mode
{cond}, if used, is always the last element of all instructions.

In addition, some flexibility is permitted that was not permitted in previous assemblers as Table 15 shows.

Table 15. Relaxation of requirements

RelaxationPermitted syntaxPreferred syntax
If the destination register is the same as the first operand, you can use a two register form of the instruction.
ADD r1, r3
ADD r1, r1, r3

You can write source code for Thumb processors earlier than ARMv6T2 using UAL.

If you are writing Thumb code for a processor earlier than ARMv6T2, you must restrict yourself to instructions that are available on the processor. The assembler generates error messages if you attempt to use an instruction that is not available.

If you are writing Thumb code for an ARMv6T2 or later processor, you can minimize your code size by using 16-bit instructions wherever possible.

Table 16 shows the main differences between the UAL and the pre-UAL Thumb assembly language.

Table 16. Differences between pre-UAL Thumb syntax and UAL syntax

ChangePre-UAL Thumb syntaxUAL syntax
The default addressing mode for LDM and STM is IALDMIA, STMIALDM, STM
You must use the S postfix on instructions that update the flags. This change is essential to avoid conflict with 32-bit Thumb-2 instructions.
ADD r1, r2, r3
SUB r4, r5, #6
MOV r0, #1
LSR r1, r2, #1
ADDS r1, r2, r3
SUBS r4, r5, #6
MOVS r0, #1
LSRS r1, r2, #1
The preferred form for ALU instructions specifies three registers, even if the destination register is the same as the first operand. However, the UAL syntax allows the two register syntax.
ADD r7, r8
SUB r1, #80
ADD r7, r7, r8
SUBS r1, r1, #80
If Rd and Rn are both Lo registers, MOV Rd, Rn is disassembled as ADDS Rd, Rn, #0.
MOV r2, r3
MOV r8, r9
CPY r0, r1
LSL r2, r3, #0
ADDS r2, r3, #0
MOV r8, r9
MOV r0, r1
MOVS r2, r3
NEG Rd, Rm is disassembled as RSBS Rd, Rm, #0.NEG Rd, RmRSBS Rd, Rm, #0

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