Convert User Input Character to Symbol: x86_64 Assembly - linux

Question

I am trying to convert an input character to an output symbol in x86 assembly. As an example, if the user enters A I want the below to be output:
$
$ $
$$$$$
$ $
$ $
And for B:
$$$$
$ $
$$$$
$ $
$$$$
etc. My current method is to set up a string of 0's and 1's and then loop through and convert a 0 to a space and a 1 to a $. I use a b as a line break (i.e. new line) and an x as end of string.
My code below (just showing A run through to shorten code):
section .data
; constants
NULL equ 0
EXIT_SUCCESS equ 0
EXIT_FAIL equ 1
SYS_exit equ 60
SYS_read equ 0
SYS_write equ 1
STD_in equ 0
STD_out equ 1
; other
text1 db "Please enter an upper-case letter from A-E: ",0
errmsg db "Error: incorrect letter chosen.",0
sucmsg db "Success.",0
symA db "00100b01010b11111b10001b10001x ",0
space db " "
dollar db "$"
lbreak db "b"
lend db "x"
section .bss
; reserve space for user input
letter resb 1
section .text
global _start
_start:
; print question
mov rax, text1
call _printText
; get user input
; sys_read (0, latter, 1)
mov rax, SYS_read
mov rdi, STD_in
mov rsi, letter
mov rdx, 1
syscall
; dereference rsi
movzx rsi, byte [letter]
; jump conditionals
mov rdx, "A"
cmp rsi, rdx
je _printA
; default jump if no match
; print fail msg
mov rax, errmsg
call _printText
jmp _exitFail
_printA:
xor eax, eax
; sys_write (1, text, 1)
mov rax, symA
call _printText
call _printChar
jmp _exitSuccess
_exitFail:
; default error exit
; sys_exit (1)
mov rax, SYS_exit
mov rdi, EXIT_FAIL
syscall
_exitSuccess:
; print success msg
mov rax, sucmsg
call _printText
; sys_exit (0)
mov rax, SYS_exit
mov rdi, EXIT_SUCCESS
syscall
; functions
_printText:
push rax
mov rbx, 0
_ptLoop:
inc rax
inc rbx
mov cl, [rax]
cmp cl, 0
jne _ptLoop
mov rax, SYS_write
mov rdi, STD_out
pop rsi
mov rdx, rbx
syscall
ret
_printChar:
;push rax
mov rbx, 0
_pcLoop:
inc rax
inc rbx
mov cl, [rax]
; if 0
cmp cl, 0
je _movSpace
; if 1
cmp cl, 1
je _movSymbol
; if newline
cmp cl, lbreak
je _movNewLine
; if end
cmp cl, lend
je _endPrint
_movSpace:
mov rcx, space
loop _pcLoop
_movSymbol:
mov rcx, dollar
loop _pcLoop
_movNewLine:
mov rcx, "\n"
loop _pcLoop
_endPrint:
mov rax, SYS_write
mov rdi, STD_out
mov rsi, rcx
mov rdx, rbx
syscall
ret
Currently, I have tried to debug as I was getting Segmentation fault (core dumped) in the _printChar function, however, now with the above code I am unable to create the executable file as it is returning the below error:
(.text+0x10e): relocation truncated to fit: R_X86_64_8 against '.data'
(.text+0x113): relocation truncated to fit: R_X86_64_8 against '.data'
when trying to run the below command in terminal:
ld filename.o -o filename
Can anyone suggest anything to assist with code above to achieve the objective I am after?

Answer 1

I assume that you make your program with
nasm -f elf64 filename.asm -o filename.o -l filename.lst
ld filename.o -o filename
Linker reports a problem at .text+0xca, so you need to find in the "filename.lst" what instruction was generated at this offset 0xca in the .text section:
....
112 000000C3 80F901 cmp cl, 1
113 000000C6 7416 je _movSymbol
114 ; if newline
115 000000C8 80F9[78] cmp cl, lbreak
116 000000CB 741D je _movNewLine
117 ; if end
118 000000CD 80F9[79] cmp cl, lend
119 000000D0 741F je _endPrint
....
It's the cmp cl, lbreak. Looking at the definition lbreak db "b" it obvious that your instruction incorrectly tried to compare a byte-value in cl with the offset of variable lbreak in section .data, which happens to be 78. You probably intended to compare cl with the value "b" stored in memory, using cmp cl, [lbreak] or even better, with the immediate value: cmp cl,"b".
Also consider printing the big letter as one 0-terminated string, using a single _printText:
SECTION .data
BigA DB " $ ",10
DB " $ $ ",10
DB "$$$$$",10
DB "$ $",10
DB "$ $",10
DB 0
BigB DB "$$$$ ",10
DB "$ $",10
DB "$$$$ ",10
DB "$ $",10
DB "$$$$ ",10
DB 0