Question

So I’m learning x86_64 nasm assembly on my mac for fun. After hello world and some basic arithmetic, I tried copying a slightly more advanced hello world program from this site and modifying it for 64 bit intel, but I can’t get rid of this one error message: hello.s:53: error: Mach-O 64-bit format does not support 32-bit absolute addresses. Here is the command I use to assemble and link: nasm -f macho64 hello.s && ld -macosx_version_min 10.6 hello.o. And here is the relevant line:

cmp rsi, name+8

rsi is the register I am using for my index in the loop, and name is a quad word reserved for the user input which is the name, which by this point has already been written.

Here is a part of the code (to see the rest, click the link and go to the bottom, the only difference is that I use 64 bit registers):

loopAgain:
mov al, [rsi]           ; al is a 1 byte register
cmp al, 0x0a            ; if al holds an ascii newline...
je exitLoop             ; then jump to label exitLoop

; If al does not hold an ascii newline...
mov rax, 0x2000004      ; System call write = 4
mov rdi, 1              ; Write to stdout = 1
mov rdx, 1              ; Size to write
syscall

inc rsi

cmp rsi, name+8         ; LINE THAT CAUSES ERROR
jl loopAgain

Answer 1

The cmp instruction does not support a 64-bit immediate operand. As such, you cannot put a 64-bit immediate address reference in one of its operands – load name+8 into a register then compare to that register.

You can see what instruction encodings are permitted in the Intel ISA manual (warning: huge PDF). As you can see on the entry for CMP, there are CMP r/m32,
imm32 and CMP r/m64,
imm32 encodings, which allow for comparisons of a 32-bit immediate with both 32-bit and 64-bit registers, but not a CMP r/m64, imm64. There is, however, a MOV r64, imm64 encoding.

Or even better, use a RIP-relative LEA: Use default rel then lea r64, [name+8]. This is more efficient and smaller than mov r64, imm64.

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Since nasm is crashing, the failure of MOV rcx, name+8 is just plain a bug in nasm. Please report it to the nasm devs (after making sure you’re using the latest version of nasm; also, check that this patch doesn’t fix the problem). In any case, though, one workaround would be to add a symbol for the end of name:

name:
    resb 8
name_end:

Now simply use MOV rcx, name_end. This has the advantage of not needing to update the referents when the size of name changes. Alternately you could use a different assembler, such as the clang or GNU binutils assemblers.

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Discussion in comments points out that Linux can use a symbol address as a 32-bit immediate. This is true only in non-PIE executables which are linked with a base address in the low 2GiB of virtual address space. But MacOS chooses to put the image base address above 4GiB so you can’t use mov r32, imm32 or cmp r64, sign_extended_imm32 with symbol addresses.