Error: no instruction for this cpu level (or, force REX prefix)

Question

I'm working on an X32 object file, which uses 32-bit Integers, Longs and Pointers. I'm building the object file like so:

nasm -f elfx32 rdrand.S -DX32 -o rdrand-x32.o

rdrand.S is multi-platofrm (X86, X32 and X64). Within the X86 block, 64-bit registers are available. So I am trying to (other code omitted to demonstrate the core problem):

%ifdef X32
  %define  buffer    rdi    ;; x32-abi (http://sites.google.com/site/x32abi/documents)
  %define  size      rsi

  rdrand   rax              ;; generate a 64-bit random value
  mov      [buffer], rax    ;; save it to caller's buffer
%else
  %define  buffer    ...

  rdrand   eax              ;; generate a 32-bit random value
  mov      [buffer], eax    ;; save it to caller's buffer
%endif

However, it appears NASM is not issuing the REX prefix on the instruction, so its resulting in:

line 100: error: no instruction for this cpu level
line 101: error: no instruction for this cpu level

I know rdrand rax will work, so its a matter of getting NASM to issue the REX prefix. (I'm not sure about mov [buffer], rax, but I'll get to that once I figure out the REX issue).

How do I tell NASM to issue the wide version of instruction?

I'm using Debian's version of NASM:

$ nasm -v
NASM version 2.11.05 compiled on Sep  9 2014

However, I'm not in an X32 enabled kernel at the moment.

My nasm version 2.10.01 correctly emits the rex prefix and has no problem with this code. Also tested with 2.11.08 that works fine, too. — Jester, Nov 10 '15 at 13:57
@Jester - I added the version information to the question. I double checked the results. What flags are you using for the assembler? — jww, Nov 10 '15 at 14:18
I used your command line. Just tested with debian 2.11.05 works too. And I am not in X32 either, although I don't think that matters. I did define `buffer` to get it to assemble. — Jester, Nov 10 '15 at 14:43

score 1 · Answer 1 · answered Nov 10 '15 at 14:46

%ifdef X32
  rdrand   rax              ;; generate a 64-bit random value
  mov      [buffer], rax    ;; save it to caller's buffer
  ...
%endif

Here's what I had to do (it takes more time to search for NASM's way of doing things than to issue it by hand):

            ;; X32 can use the full register, issue the REX prefix
%ifdef X32
    DB 48h
%endif
            ;; RDRAND is not available prior to Clang 3.2. Just emit
            ;;   the byte codes using DB. This is `rdrand eax`.
    DB 0Fh, 0C7h, 0F0h

And then to move the bits from rax:

%ifdef X32
    mov        [buffer+4], eax    ;; We can only move 4 at a time
    DB         048h               ;; Combined, these result in 
    shr        eax, 32            ;;   `shr rax, 32`
%endif

    mov        [buffer], eax
    add        buffer, 8          ;; No need for Intel Core 2 slow word workarounds,
    sub        bsize, 8           ;;   like faster adds using `lea buffer,[buffer+8]`

Error: no instruction for this cpu level (or, force REX prefix)

1 Answers1