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 12 highlights the differences between the UAL and pre-UAL ARM assembly language syntax.

LDMIA, STMIA

STMFD sp!, {reglist}
LDMFD sp!, {reglist}

PUSH {reglist}
POP {reglist}

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

LDR Rd, [pc, #offset]

LDR Rd, label

LDRD Rd, addr_mode

LDRD Rd, Rd2, addr_mode

ADD{cond}S
LDR{cond}SB

ADDS{cond}
LDRSB{cond}

In addition, some flexibility is permitted that was not permitted in previous assemblers as Table 13 shows.

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

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

ADD r7, r8
SUB r1, #80

ADD r7, r7, r8
SUBS r1, r1, #80

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