On 21/06/11 06:21, Jeffrey Walton wrote: > Hi All, > > ARMv6, -mthumb, and Apple's AS (1.38, probably modified). > > I've got a routine to add two unsigned ints and then check the carry > flag in the CPSR. To perform the add, I use the Thumb-16 ADD > instruction (ADDS is not available). > > __asm__ volatile > ( > "ldr r4, %[xa] ;" // R4 = a > "ldr r5, %[xb] ;" // R5 = b > "add r6, r4, r5 ;" // R6 = R4 + R5, set status > "bcc 1f ;" // jump if carry is clear > "mov r5, #0 ;" // set overflow > "str r5, %[xc] ;" // write it to memory > "1: ;" // jump label > "str r6, %[xr] ;" // result = R6 > : [xr] "=m" (result), [xc] "=m" (no_carry) > : [xa] "m" (a), [xb] "m" (b) > : "r4", "r5", "r6" > ); > > According to 'ARM® Developer Suite, v1.2', Section 2.2.8, page 2-9, > the status flags are set with ADD under Thumb-16: > > The conditional branch instruction is the only Thumb > instruction that can be executed conditionally on the > value of the ALU status flags in the CPSR. All data > processing instructions update these flags, except > when one or more high registers are specified as > operands to the MOV or ADD instructions. In these > cases the flags cannot be updated. > > Unfortunately, the status flags are not being updated. Yes they are, as you'll see when you step through. This is a logic failure: your asm declares that it writes no_carry, but it only does so if carry is set. Your asm must also write to no_carry when carry is clear. Then your asm will work. Andrew.
