No "beq" or "bne" instruction in ARMv7-M manual?

Question

I'm working on a STM32l475 micro-controller which runs a Cortex-M4 processor and ARM/Thumb instruction sets. I see (from objdump) that there are beq.n and bne.n instructions generated in the binary of an ARM program (I added the -mthumb flag when compile the program). However, I don't find these branch instructions in the latest ARMv7-M manual.

Can anyone tell me the reason? And what are the instructions available in the manual that are equivalent to these two branch instructions?

Fitch

Answer 1

beq and bne are conditional branches; in other words, they are conditional versions of the unconditional branch b. eq and ne are two different condition codes; they are described in section A7.3. beq means branch if equal and bne means branch if not equal.

The b branch instruction has two different encodings in Thumb mode. The encoding you're seeing is probably encoding T1 described in section A7.7.12:

B<c> <label>

15 14 13 12 11 10  9  8  7  6  5  4  3  2  1  0
 1  1  0  1 [-<cond>--] [--------imm8---------] 

In this encoding, the condition code (like eq or ne) is encoded directly into the instruction, in bits 8-11. The disassembly from objdump displays the condition code in place of <c> above. So using the condition code table in section A7.3, you would encode beq as 11010000[imm8].

Blakeley