Answer:
Here's an example program written in NASM x86 assembly that reads one byte of input and prints a message depending on whether the input is a B or not:
section .data
message_b: db "It's a B",10
message_not_b: db "It's NOT a B",10
section .bss
input_buffer: resb 1
section .text
global _start
_start:
; read one byte of input from stdin
mov eax, 3
mov ebx, 0
mov ecx, input_buffer
mov edx, 1
int 0x80
; check if input is B
cmp byte [input_buffer], 'B'
jne not_b
; print "It's a B" message
mov eax, 4
mov ebx, 1
mov ecx, message_b
mov edx, 8
int 0x80
jmp end
not_b:
; print "It's NOT a B" message
mov eax, 4
mov ebx, 1
mov ecx, message_not_b
mov edx, 12
int 0x80
end:
; exit program
mov eax, 1
xor ebx, ebx
int 0x80
Step-by-step explanation:
The program first declares two messages, one for when the input is a B and another for when it's not. It then reserves one byte of memory for the input buffer using the .bss section.
In the .text section, the program first reads one byte of input from stdin using the read system call. It then compares the input to the ASCII value of the letter B using the cmp instruction. If the input is not B, it jumps to the not_b label to print the "It's NOT a B" message. If the input is B, it continues to print the "It's a B" message.
The program uses the mov instruction to load the appropriate values into the registers required by the write system call. The message to be printed is stored in the ecx register, while the length of the message is stored in the edx register. The int 0x80 instruction is then used to invoke the write system call and print the message to stdout.
Finally, the program exits using the exit system call by loading the value 1 into the eax register and calling int 0x80.