Question

I have a program reading characters in raw mode. That is, any characters input are read immediately instead of being buffered.

I would like to know how to perform a backspace. That is, when I press the Backspace key, it should delete the character on the left and move the cursor one place to the left.

I have tried outputting a Backspace character followed by a Space character. This deletes the character on the left, but moves the cursor two spaces to the right, for some reason.

I have also tried outputting the Backspace character by itself. This moves the cursor one space to the left, but doesn’t delete the character.

My assembler is YASM and I am using 64-bit Linux. Below is some relevant code.

First, here’s the function I use to output a character:

printchar:                       
        mov     [buf], al      ; Backup whatever was in al.
                               ; buf is declared as resb 1 in section .bss
        mov     edx, 1
        mov     ecx, buf
        mov     ebx, 1
        mov     eax, 4
        int     0x80
        mov     al, [buf]    ; restore char that was previously in al
        ret

The following code prints a Backspace. It doesn’t move the cursor one place to the left, like your regular Backspace does.

    mov     al, 0x08 ; ASCII for Backspace
    call    printchar

This code prints a Backspace followed by a Space. It moves the cursor TWO spaces to the right instead of just one.

    mov     al, 0x08 ; ASCII for Backspace
    call    printchar
    mov     al, 0x20 ; ASCII for Space
    call    printchar

Len suggested printing a Backspace, followed by a Space, followed by a Backspace. This seems to work:

    mov     al, 0x08 ; ASCII for Backspace
    call    printchar
    mov     al, 0x20 ; ASCII for Space
    call    printchar
    mov     al, 0x08 ; ASCII for Backspace
    call    printchar

Please note that, as I previously mentioned, I am reading characters in raw mode. Thanks in advance!

Answer 1

I have also tried outputting the Backspace character by itself. This moves the cursor one space to the left, but doesn’t delete the character.

You’re almost there! Try outputting a backspace, then a regular space to black out the character the cursor is now on top of, then another backspace to move the cursor back once again.

It seems like a hack, but it’s actually quite common!