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2.11. Assembly language changes

Table 2.10 shows the main differences between the current ARM/Thumb assembler language and the earlier separate ARM and Thumb assembler languages. The old ARM syntax is accepted by the assembler.

Table 2.10. Changes from earlier ARM assembly language

ChangeOld ARM syntaxPreferred syntax
The default addressing mode for LDM and STM is IA
LDMIA, STMIA
LDM, STM
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
MOV Rd, Rn, RRX

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.
ADD{cond}S
LDR{cond}SB

ADDS{cond}
LDRSB{cond}
You can use both ARM {cond} conditional forms and Thumb-2 IT instructions, in both ARM and Thumb-2 code. The assembler checks for consistency between the two, and assembles the appropriate code depending on the current instruction set.
ADDEQ r1, r2, r3
LDRNE r1, [r2, r3]

ITEQ E
ADDEQ r1, r2, r3
LDRNE r1, [r2, r3]

In addition, some flexibility is permitted that was not permitted in previous assemblers (see Table 2.11).

Table 2.11. Relaxation of requirements

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

ADD r1, r3

You can write source code for Thumb processors using the ARM/Thumb assembler language.

If you are writing Thumb code for a pre-Thumb-2 processor, 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 a Thumb-2 processor, you can minimize your code size by using 16-bit instructions wherever possible.

Table 2.12 shows the main differences between Thumb assembly language and ARM assembly language. The assembler accepts the old Thumb syntax only if it is preceded by a CODE16 directive, or if the source file is assembled with the --16 command-line option.

Table 2.12. Differences between old Thumb syntax and current syntax

ChangeOld Thumb syntaxNew 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.
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
NOP instructions replace MOV r8, r8 when available.- NOPNOP MOV r8, r8

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