I'm trying to print a floating-point value from assemler calling a printf function. It works fine with strings and integer values but fails printing floats. Here is an example of working code:
global main
extern printf
section .data
message: db "String is: %d %x %s", 10, 0
end_message: db ".. end of string", 0
section .text
main:
mov eax, 0xff
mov edi, message
movsxd rsi, eax
mov rdx, 0xff
mov rcx, end_message
xor rax, rax
call printf
ret
String is: 255 ff .. end of string
So, the parameters are passed through registers: edi contains address of a formatting string, rsi and rdx contain the same number to print in decimal and hex styles, rcx contains end of a string, rax contains 0 as we do not have a float to print. This code works fine but something changes while trying to print float:
global main
extern printf
section .data
val: dq 123.456
msg: db "Result is: %fl",10, 0
section .text
main:
mov rdi,msg
movsd xmm0,[val]
mov eax,1
call printf
mov rax, 0
ret
This code snipped can be compiled but returns segmentation fault being executed. It seems that the problem is in wrong value of xmm0 but trying to change movsd xmm0,[val] to movsd xmm0,val gives an
error: invalid combination of opcode and operands
message. The compiler is NASM running on openSuSe 12.3
Update. I tried to make a c program and produce a .S assembly. It gives a very weird solution:
main:
.LFB2:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $32, %rsp
movl %edi, -4(%rbp)
movq %rsi, -16(%rbp)
movq val(%rip), %rax
movq %rax, -24(%rbp)
movsd -24(%rbp), %xmm0
movl $.LC0, %edi
movl $1, %eax
call printf
movl $0, %eax
leave
.cfi_def_cfa 7, 8
ret
Is it possible to write a simple printf example?
