Question

I m trying to learn MIPS Assembly by learning MIPS Assembly Language Programming. In the book I have this code(extracted from the page 37 of the book):

.data
prompt: .asciiz "
 Please Input a Value: "
bye: .asciiz "
 Bye!"
.globl main
.text

main:
   li $v0, 4
   la $a0, prompt
   syscall

   li $v0, 5
   syscall
   beqz $v0, end
   move $a0, $v0
   li $v0, 1
   syscall
   b main


end:
   li $v0, 4
   la $a0, bye
   syscall

   li $v0, 10
   syscall

I have a cross-compiled binutils targeted to mips-elf, but when I ve tried to Assemble the code, I got some errors

ubuntu@eeepc:~/Desktop$ mips-elf-as test-mips.asm
test-mips.asm: Assembler messages:
test-mips.asm:8: Error: illegal operands li
test-mips.asm:9: Error: illegal operands la
test-mips.asm:12: Error: illegal operands li
test-mips.asm:14: Error: illegal operands beqz
test-mips.asm:15: Error: illegal operands move
test-mips.asm:16: Error: illegal operands li
test-mips.asm:22: Error: illegal operands li
test-mips.asm:23: Error: illegal operands la
test-mips.asm:26: Error: illegal operands li
ubuntu@eeepc:~/Desktop$

I m using x86 Ubuntu Hardy Herron to cross-compile to MIPS

What is wrong?

Answer 1

I think the problem is that you re using an older version of binutils which does not support the symbolic names for MIPS registers.

binutils 2.17 (as referenced in the cross-compilation instructions you linked to) does not understand $v0, $a0 etc. (see this question).

However, if you are building the tools yourself anyway, a good solution would be to move to a later version of binutils: versions from 2.18 onwards do support symbolic register names. Your example assembles correctly with either 2.18 or the latest version, 2.20.

Answer 2

Well, the li and la instructions are pseudo instructions which should be recognised by the assembler but it s possible your environment needs to do something to define them (it wouldn t surprise me if they were macros).

Try changing them to their "real" form:

li $v0,4   -->   lui $v0,0;       ori $v0,$v0,4
la $a0,bye -->   lui $a0,bye>>16; ori $a0,$a0,bye&ffff

Or whatever your particular MIPS assembler uses for RIGHT-SHIFT-16-BITS and AND.

The ls instruction I ve never seen before. Are you sure that s not a typo for la? And I think beqz should be beq. Most CPUs would use the same instruction in different ways, an example being that both decrementing a register down to zero (z) and comparing a register with something (eq) would set the zero flag.

None of ls, beqz or move show up in the MIPS assembler Wikipedia page although the latter two are listed as pseudo-instructions in Patterson & Hennessy.

So it s looking more and more like there s some extra set-up you need to do to get the pseudo-instructions working.

Answer 3

I think it is your sequence of listing of li and la, unless you load an address, you can t load immediate and tell $v0 to hold on to the string you want to appear.. try putting the la first before li in each case... well as for pseudo instructions, yes, that can be the problem as well, but my suggestion is worth a try.

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