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Question about T3 and T4 encoding of B/Bcc (Branch) Instruction

WestfW over 5 years ago

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?

Parents

0 WestfW over 5 years ago in reply to WestfW

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

Reply

0 WestfW over 5 years ago in reply to WestfW

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

Children

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