Building a Command Table in Assembly | Beginner Tutorial | x86-64 Assembly Programming Course
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CODE TO COPY/PASTE:
;###############
;# CODE START
;##################
.intel_syntax noprefix
.global _start
.text
_start:
lea rdi, [buf]
lea rsi, [bufsize]
call readline
cmp rax, 0
je _start
lea rdi, [buf]
lea rsi, [tokens]
call tokenize
lea rdi, [tokens]
mov rdi, [rdi]
lea rsi, [s_cmd_exit]
call strcmp ;# rax == 0 if strings match
je exit
lea rdi, [tokens]
mov rdi, [rdi]
lea rsi, [s_cmd_echo]
call strcmp
je select.echo
lea rdi, [tokens]
mov rdi, [rdi]
lea rsi, [s_cmd_help]
call strcmp
je select.help
lea rdi, [s_nocommand]
call println
jmp _start
select.help:
call help
jmp _start
select.echo:
lea rdi, [tokens + 8]
call echo
jmp _start
help:
lea rdi, [s_help]
call print
ret
echo:
mov r15, rdi
echo.loop:
mov rdi, [r15]
cmp rdi, 0
je echo.ret
call print
lea rdi, [space]
call print
add r15, 8
jmp echo.loop
echo.ret:
lea rdi, [newline]
call print
ret
strcmp: ;# args: rdi s1, rsi s2
xor rax, rax
mov al, [rdi]
mov dl, [rsi]
cmp al, dl
jne sc.done
cmp al, 0
je sc.done
inc rdi
inc rsi
jmp strcmp
sc.done:
sub al, dl
ret
strncmp: ;# args: rdi s1, rsi s2, rdx len
mov rcx, rdx
snc.loop:
dec rcx
mov al, [rdi]
mov dl, [rsi]
cmp al, dl
jne snc.done
cmp al, 0
je snc.done
cmp rcx, 0
je snc.done
inc rdi
inc rsi
jmp snc.loop
snc.done:
sub al, dl
ret
nodata:
lea rdi, [s_nodata]
call print
call exit
print_tokens:
lea r15, [tokens]
pt.loop:
mov rdi, [r15]
cmp rdi, 0
je pt.end
call println
add r15, 8
jmp pt.loop
pt.end:
ret
tokenize: ;# arg1: rdi (string), arg2: rsi (arr)
mov r15, rsi
tokenize.loop:
call skipwhite
cmp byte ptr [rdi], 0x0
je tokenize.end
mov [r15], rdi
add r15, 8
call findend
cmp byte ptr [rdi], 0
je tokenize.end
mov byte ptr [rdi], 0
inc rdi
jmp tokenize.loop
tokenize.end:
mov qword ptr [r15], 0
ret
findend:
cmp byte ptr [rdi], ' '
je findend.ret
cmp byte ptr [rdi], 0
je findend.ret
inc rdi
jmp findend
findend.ret:
ret
skipwhite:
cmp byte ptr [rdi], ' '
je skipwhite.advance
ret
skipwhite.advance:
inc rdi
jmp skipwhite
exit:
xor rdi, rdi
mov rax, 60
syscall
print:
push rdi
call slen
pop rsi
mov rdx, rax
mov rax, 1
mov rdi, 1
syscall
ret
println:
push rdi
call slen
pop rsi
mov rdx, rax
mov rax, 1
mov rdi, 1
syscall
mov rax, 1
mov rdi, 1
lea rsi, [newline]
mov rdx, 1
syscall
ret
slen:
xor rcx, rcx
slen.loop:
mov al, [rdi + rcx]
cmp al, 0
je slen.ret
inc rcx
jmp slen.loop
slen.ret:
mov rax, rcx
ret
readline:
lea rdx, [rsi - 1]
mov rsi, rdi
mov rax, 0
mov rdi, 0
syscall
mov byte ptr [rsi + rax], 0
ret
memview:
mov r14, rsp
lea rsp, [buf]
int3
mov rsp, r14
ret
.data
buf: .skip 128, 0xaa
bufsize = . - buf
tokens: .skip 128, 0xbb
newline: .asciz "\n"
space: .asciz " "
s_nodata: .asciz "\nNo data entered!\n"
s_nocommand: .asciz "\nUnsupported command!\n"
s_cmd_exit: .asciz "exit"
s_cmd_echo: .asciz "echo"
s_cmd_help: .asciz "help"
s_help: .asciz "\nSupported commands:\n1 echo\n2 exit\n3 help (this function)\n"
;#########
;# CODE END
;#########
00:00 our tiny shell
1:00 parsed input
02:14 making our command table
03:18 visual look at our array
04:11 adding our loop
06:35 incrementing per element
07:05 selecting the function pointer member
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