On Fri, 7 Jun 2013, Alexis BRENON wrote: > To create the JIT, I have to load some MIPS instruction directly in memory > without passing through a .asm file or else. So, I cannot set some labels. So > to make some branches, I try to load the equivalent instruction of : > bne $t0, $t1, -8 > to go back, just before the bne instruction, if $t0 and $t1 are equals. But > when it run, I've got an illegal instruction error. Please note that BNE means Branch-if-Not-Equal, your quoted instruction will jump backwards if $t0 and $t1 are *not* equal. > To debug, I write a small program in the MARS MIPS simulator with this > instruction. But when compiling, assembler says me that -8 is an operand of > incorrect type. The instruction you quoted assembles for me successfully, what version of binutils do you use and what exact error message do you get? Please note however that this instruction is not what I understand you need -- it is treated as a branch to the absolute address -8 (0xfffffff8 in the o32 ABI), rather than 8 bytes back (there's an off-by-four bug in GAS here too making it jump to -4 instead, and some other issues; I'll see if I can get them fixed sometime -- see the discussion around http://sourceware.org/ml/binutils/2012-09/msg00288.html if interested in the gory details). If you want to jump to the instruction immediately preceding the branch and avoid a label (assuming the standard MIPS ISA), use: bne $t0, $t1, . - 4 -- "." is a special "the address of this instruction" designator (see the GAS manual for further information), so this produces the machine instruction you require (the jump is calculated as relative to the next instruction -- that is (. + 4) -- so the ultimate effective (i.e. shifted rather than as encoded in the instruction's 16-bit immediate operand field) offset is -8). $ cat foo.s bne $t0, $t1, . - 4 $ mips-linux-as -o foo.o foo.s $ mips-linux-objdump -dr foo.o foo.o: file format elf32-tradbigmips Disassembly of section .text: 00000000 <.text>: 0: 1509fffe bne t0,t1,0xfffffffc 4: 00000000 nop ... $ mips-linux-as --version GNU assembler (GNU Binutils) 2.23.2 [...] Likewise with the current binutils trunk. I hope this helps. Maciej
