I'm totaly new to assembly language. I tried to make simple program to decipher a Caesar cipher. The problem is that after linking I get these errors:
cezar.o: In function `loop':
(.text+0xbf): relocation truncated to fit: R_X86_64_8 against `.data'
cezar.o: In function `check_letter':
(.text+0xd5): relocation truncated to fit: R_X86_64_8 against `.data'
I'll be honest - I have no idea what this means. Can anybody help me? Rather, #explainMeLikeI'm5 ;P
My code if neded:
.data
STDIN = 0
STDOUT = 1
SYSWRITE = 0
SYSREAD = 1
SYSEXIT = 60
EXIT_SUCCESS = 0
BUFLEN = 512
BASE = 10
BASE_NUM = 10
BASE_CHAR = 26
NUM_BEG = 48
error_msg: .ascii "Bad characters in key\n"
error_msg_len = .-error_msg
key: .ascii "-128"
key_len = .-key
.bss
.comm textin, 512
#.comm keyin, 512
.comm textout, 512
.text
.globl _start
_start:
#check if key is good
movq $0, %rdi
movb key(, %rdi, 1), %bl #load first char to bl
cmp $'-', %bl #check if it's -
jne error
movq $1, %rdi #counter
movb key(, %rdi, 1), %bl
cmp $'0', %bl
jl error
cmp $'9', %bl
jg error
mov $key_len, %rdi
mov $1, %r11 #powers of 10
mov $0, %r8 #calculated key goes to r8
sub $1, %rdi #because I want numbers only, "-" doesn't interest me now
ascii_to_num:
cmp $1, %rdi #at rdi=0 jump out of loop
jl load_text
mov $0, %rax #clearing rax
mov key(, %rdi, 1), %al
sub $NUM_BEG, %al
mul %r11 #multiplying al by power of it's position
add %rax, %r8
mov %r11, %rax
mov $10, %r12
mul %r12
mov %rax, %r11
dec %rdi #decrementation
jmp ascii_to_num
load_text:
movq $SYSREAD, %rax
movq $STDIN, %rdi
movq $textin, %rsi
movq $BUFLEN, %rdx
dec %rax #delete '\n'
movq %rax, %r10
movq $0, %rdi #counter
loop:
movb textin(, %rdi, 1), %bh
#check if number
movb $'0', %bl #I'm using bl for comparing
cmp %bl, %bh
jl ending_loop
movb $'9', %bl
cmp %bl, %bh
jg check_letter
add $key, %bh
cmp $'0', %bh
jl correct_num
jmp ending_loop
check_letter:
movb $'A', %bl
cmp %bl, %bh
jl ending_loop
movb $'Z', %bl
cmp %bl, %bh
jg ending_loop
add $key, %bh
cmp $'A', %bh
jl correct_char
jmp ending_loop
correct_num:
add $BASE_NUM, %bh
cmp $'0', %bh
jl correct_num
jmp ending_loop
correct_char:
add $BASE_CHAR, %bh
cmp $'A', %bh
jl correct_char
jmp ending_loop
ending_loop:
movb %bh, textout(, %rdi, 1)
inc %rdi
cmp %r10, %rdi
jle loop
jmp show
error: #error message
mov $SYSWRITE, %rax
mov $STDOUT, %rdi
mov $error_msg, %rsi
mov $error_msg_len, %rdx
syscall
show: #display message and close programm
add $'\n', textout
mov $SYSWRITE, %rax
mov $STDOUT, %rdi
mov $textout, %rsi
mov $BUFLEN, %rdx
syscall
mov $SYSEXIT, %rax
mov $EXIT_SUCCESS, %rdi
syscall
The errors refer to the two lines where you wrote:
add $key, %bh
This tries to add the value of key (i.e. the address of the string -128) to bh. As the address is larger than 255, it does not fit into a byte register like bh. For this reason, you get the error indicated by the linker.
I have not analyzed what you try to do by loading the address of key into bh, but doing so is not going to work. If you want to load the first byte of the data at key, write
add key(%rip), %bh
If you want to load the address of key, you need to load it into a 64 bit register using lea like this:
lea key(%rip), %rbx