Assembly: how to convert number into ascii and write to display buffer - linux

Question

I am new to assembly and am programming in linux 64 bit in AT&T syntax. If I store the number 1 in a register, how can I translate that to the ascii character "A"? For example:
movl $1, %ebx
addl $64, %ebx
Can I add 64 to 1 to make 65 (the decimal value of A), then somehow convert it to "A" and send this to the buffer using write system call?
EDIT 1: Posting my program code here.
.section .data
message:
.long 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
length:
.long 10
.section .text
.globl _start
_start:
xorq %rdi, %rdi
xorq %rax, %rax
xorq %rbx, %rbx
xorq %rcx, %rcx
xorq %rdx, %rdx
movl length, %edx
loop:
cmpl %ecx, %edx
je loop_end
movl message(,%rdi,4), %eax
addl $64, %eax
pushq %rax
incq %rdi
incq %rcx
jmp loop
loop_end:
cmpq $0, %rcx
je exit
popq %rbx
pushq %rcx
movq $1, %rax
movq $1, %rdi
movq %rbx, %rsi
movl length, %edx
syscall
popq %rcx
decq %rcx
jmp loop_end
exit:
movq $60, %rax
movq $0, %rdi
syscall

Answer 1

I'm not entirely familiar with AT&T syntax, but the disassembly of NASM in what you're accustomed to should suffice.
You should try to avoid what is called hard coding constants as it makes your program harder to maintain, especially when it's hundreds if not thousands of lines in length. Therefore;
section .data
Values: db 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 26, 18, 12, 20, 19, 11
V_Size equ $ - Values
is preferable to this
message:
.long 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
length:
.long 10
What you did is not wrong, but the method is predicated upon you counting, not the assembler. As it has already been pointed out, use the smallest data size required to get the job done. In this case char is better than long
This code in NASM
section .text
global _start
_start: xor ecx, ecx
push rcx ; Applications default return value
mov cl, V_Size
push rcx
mov ebx, Values
push rbx
Next:
or byte [ebx], 64
inc ebx
loop Next
pop rsi
pop rdx
pop rax
inc al
mov edi, eax
syscall
mov edi, eax
dec edi
mov eax, edi
mov al, 60
syscall
section .data
Values: db 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 26, 18, 12, 20, 19, 11
V_Size equ $ - Values
will yield
ABCDEFGHIJZRLTSK
with command prompt immediatly after "K".
section .data:
6000d8 01020304 05060708 090a1a12 0c14130b
section .text:
<_start>: These two instructions are idiosyncratic to my style of programming and not
essential to functionality of program.
4000b0: 31 c9 xor %ecx,%ecx
4000b2: 51 push %rcx
Setup RCX & RBX for LOOP instruction
4000b3: b1 10 mov $0x10,%cl
4000b5: 51 push %rcx ARG2 to syscall
4000b6: bb d8 00 60 00 mov $0x6000d8,%ebx
4000bb: 53 push %rbx ARG1 to syscall
<Next>: This conforms to the scope of your objective.
4000bc: 67 80 0b 40 orb $0x40,(%ebx) [ebx] += 'A'
4000c0: ff c3 inc %ebx
4000c2: e2 f8 loop 4000bc <Next>
ssize_t write (int fd, const void *buf, size_t count);
4000c4: 5e pop %rsi ARG1 = ASCII Pntr
4000c5: 5a pop %rdx ARG2 = # of chars
4000c6: 58 pop %rax
4000c7: fe c0 inc %al SYS_WRITE
4000c9: 89 c7 mov %eax,%edi ARG0 = STD_OUT
4000cb: 0f 05 syscall
Epilogue: Again, just a method I use.
4000cd: 89 c7 mov %eax,%edi
4000cf: ff cf dec %edi
4000d1: 89 f8 mov %edi,%eax
4000d3: b0 3c mov $0x3c,%al
4000d5: 0f 05 syscall