I've got the following ARM assembly code.
CMP R0, #0 ITT EQ MOVEQ R0, #0x7FFFFFFF BXEQ LR
Firstly, why is the EQ needed after the MOV and BX instructions? The ARM reference says that the condition (EQ) after the ITT will be applied to the first instruction (MOV) in the IT block and then because of the second T in ITT the EQ will be applied to the second instruction (BX) in the IT block. So if the ITT is applying the EQ, why is the EQ needed in MOVEQ and BXEQ?
Secondly, why is the IT instruction needed at all? Why not just have:
CMP R0, #0 MOVEQ R0, #0x7FFFFFFF BXEQ LR
It's MOV not MOVS so the flags won't be updated and the EQ in BXEQ will still be "referring" to the flag values set by the CMP.
Why dont you just try it?
.cpu cortex-m3 .thumb .syntax unified CMP R0, #0 ITT EQ MOVEQ R0, #0x7FFFFFFF BXEQ LR CMP R0, #0 MOVEQ R0, #0x7FFFFFFF BXEQ LR
arm-none-eabi-as vectors.s -o vectors.o vectors.s: Assembler messages: vectors.s:13: Error: thumb conditional instruction should be in IT block -- `moveq R0,#0x7FFFFFFF' vectors.s:14: Error: thumb conditional instruction should be in IT block -- `bxeq LR' make: *** [vectors.o] Error 1
Which is obvious because there are no conditional versions of those instructions in thumb mode.
so that leaves:
.cpu cortex-m3 .thumb .syntax unified CMP R0, #0 ITT EQ MOVEQ R0, #0x7FFFFFFF BXEQ LR
which the tools are happy with
0: 2800 cmp r0, #0 2: bf04 itt eq 4: f06f 4000 mvneq.w r0, #2147483648 ; 0x80000000 8: 4770 bxeq lr
so we try without the eq
.cpu cortex-m3 .thumb .syntax unified CMP R0, #0 ITT EQ MOV R0, #0x7FFFFFFF BX LR
vectors.s:8: Error: instruction not allowed in IT block -- `mov R0,#0x7FFFFFFF' vectors.s:9: Error: incorrect condition in IT block -- `bx LR'
I think it must be just a syntax thing to help you out and make sure that you get what you really wanted.
.cpu cortex-m3 .thumb .syntax unified CMP R0, #0 IT EQ MOVEQ R0, #0x7FFFFFFF BX LR
0: 2800 cmp r0, #0 2: bf08 it eq 4: f06f 4000 mvneq.w r0, #2147483648 ; 0x80000000 8: 4770 bx lr
Notice the bx lr is the same instruction 0x4770, the eq on the end or not on the end seems clearly there as an assembler syntax thing to help you out and make sure you get the right number of instructions tied to the If Then instruction. (which you can see did change between having one conditional instruction and two conditional instructions).
I do find it bothersome
.cpu cortex-m3 .thumb .syntax unified CMP R0, #0 IT EQ MOVSEQ R0, #0x7 BX LR movs r0,#7 mov r0,#7 movs.w r0,#7
that in this case the thumb2 extension is used
00000000 <.text>: 0: 2800 cmp r0, #0 2: bf08 it eq 4: f05f 0007 movseq.w r0, #7 8: 4770 bx lr a: 2007 movs r0, #7 c: f04f 0007 mov.w r0, #7 10: f05f 0007 movs.w r0, #7
that is a curiosity.
The reason it is needed at all is obvious from the instruction set documentation. Full blown arm instructions have a 4 bit conditional field on every instruction. thumb instructions do not. At first you simply did the traditional branch on condition to avoid instructions, thumb didnt offer the ARM feature of every instruction being conditional and not needing to flush the pipe. So according to the docs they added the If Then (IT) instruction with ARMv7-M, and as stated in those docs this allows you to make up to four instructions following the if then to become conditional. The above syntax game I believe (have no proof other than it just appears to be so) is there to help on the human error.
Now if not in thumb mode then you absolutely can just apply the conditional to the instruction
.syntax unified CMP R0, #0 MOVSEQ R0, #0x7 BXEQ LR movs r0,#7 mov r0,#7
00000000 <.text>: 0: e3500000 cmp r0, #0 4: 03b00007 movseq r0, #7 8: 012fff1e bxeq lr c: e3b00007 movs r0, #7 10: e3a00007 mov r0, #7
and maybe this is the root of your question, but it is very possible that the assembler could just insert the IT instruction for you, but assembly language has a desire to be one to one (despite all the pseudo instructions for all the processors that are out there) so I guess they expect you to explicitly show you want that If Then instruction there and/or that you are going to have an If Then instruction there. The assembler is also helping you by saying you need to use an IT block rather than simply saying it is an invalid instruction.
One further experiment
.cpu arm7t .thumb .syntax unified CMP R0, #0 MOVSEQ R0, #0x7 BX LR movs r0,#7
Is bothersome because if you leave the IT in there it knows that is wrong:
vectors.s:7: Error: selected processor does not support Thumb mode `it EQ'
but then in the same breath it says
vectors.s:7: Error: thumb conditional instruction should be in IT block -- `movseq R0,#0x7'
The ARMv7-A and ARMv7-M edition of the ARM Architecture Reference (A4.2.1 "Conditional instructions) says this:
Although other Thumb instructions are unconditional, all instructions that are made conditional by an IT instruction must be written with a condition. These conditions must match the conditions imposed by the IT instruction. For example, an ITTEE EQ instruction imposes the EQ condition on the first two following instructions, and the NE condition on the next two. Those four instructions must be written with EQ, EQ, NE and NE conditions respectively.
I agree with dwelch that it's likely specified this way to help reduce programming errors, as the condition code isn't encoded in the machine opcode.
Also, for the purpose of the 'unified assembler language' (where the same assembly mnemonics can be used for either 32-bit ARM or Thumb modes), the opposite is done in ARM mode. The
IT instructions are checked for consistency with the conditional instructions that follow even though there is no machine opcode generated for the
For maximum portability of UAL assembly language between the ARM and Thumb instruction sets, ARM recommends that:
IT instructions are written before conditional instructions in the correct way for the Thumb instruction set.
When assembling to the ARM instruction set, assemblers check that any IT instructions are correct, but do not generate any code for them.
You need the
IT instruction in thumb mode, where condition bits are not available in the instructions otherwise. Your first example is most probably thumb code and not ARM mode.
why is the EQ needed in MOVEQ and BXEQ?
You can use the inverse condition in an IT block. I think it is also easier to read this way.
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