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On Cortex v7M architecture, looking at the T3 and T4 encodings for the B and Bcc instructions in the architecture reference manual...
Do T3 and T4 really have the J bits of the immediate value in the opposite order from one another?
T3 (20 bit immediate value): imm32 = SignExtend(S:J2:J1:imm6:imm11:'0', 32);T4 (24bit immediate value): I1 = NOT(J1 EOR S); I2 = NOT(J2 EOR S); imm32 = SignExtend(S:I1:I2:imm10:imm11:'0', 32);All the references are consistent, but it just seems particularly odd.Is there some more intuitive way to look at the Not(S xor Jn) part of the instructions?
T3 (20 bit immediate value): imm32 = SignExtend(S:J2:J1:imm6:imm11:'0', 32);
T4 (24bit immediate value): I1 = NOT(J1 EOR S); I2 = NOT(J2 EOR S); imm32 = SignExtend(S:I1:I2:imm10:imm11:'0', 32);
I don't think I trust my assembler (gas):
I mean, at least two of these are wrong, right?
Disassembly of section .text: 00000000 <start0>: 0: f47f affe bne.w 9e1ea <*ABS*0x9e1ea> 4: f47f affe bne.w 7fffe <*ABS*0x7fffe> 8: f47f affe bne.w fffff <*ABS*0xfffff>
Ah; sorry. that seems just incompletely linked, for all it produced an a.out :-(
Here's correct assembly/disassembly/hand decode. It's hard to tell - the J bits look like they're in the same order, but the unconditional branch case has that possible inversion going on...
Source (both have J1:J2 == 10, hopefully): @ SJJimm6..imm11..89ABw bne .+0b010011110000111101010 + 4 @ SJJimm10..89Aimm11..89ABw b .+0b0100111100001111011001010 + 4 Disassembly and hand decode: 00008000 <_start>: @ 8000: f05e 88f5 bne.w a61ee <_stack+0x261ee> @ oooo oScc ccii iiii ooJo Jiii iiii iiii @ 1111 0000 0101 1110 1000 1000 1111 0101 @ 8008: f1e1 9f65 b.w 9e9ed6 <*ABS*0x13c3d4+0x8adb02> @ oooo oSii iiii iiii ooJo Jiii iiii iiii @ 1111 0001 1110 0001 1001 1111 0110 0101
WestfW said: don't think I trust my assembler (gas):
*rotfl*
It would be serious if "gas" produces wrong op-codes from such a standard mnemonics.
True. I probably meant "I'm not sure I understand how to get the assembler to generate the instructions I think it should." The format above "B . + immConstant*2 + 4" seems to be it...Although, you could go a long time with errors in the encoding or documentation of the J bits for B instruction on most Cortex-M processors - that's the range beyond the amount of flash on most chips, and not enough to get to RAMFUNCs.
T3/T4 encoding applies to all Thumb-2 Cores, means Cortex-A,R and M. So T4 might be used. Still not clear to me, where your problem lies. The encoding of T4 is kind of weird, agree. But I fear, you won't get an answer to the "why" :-)