Print the contents of the CR0 register

Issue

I am making my own bootloader which switchs to protected mode(32-bits) and then prints the contents of the CR0 register (the one used to turn on protected mode). I need to write the program on assembly.

    mov esi,hello
    mov ebx,0xb8000
.loop:
    lodsb
    or al,al
    jz halt
    or eax,0x0100
    mov word [ebx], ax
    add ebx,2
    jmp .loop
halt:
    cli
    hlt
hello: db "Hello world!",0

Solution

This will show CR0 in binary representation. It uses the same output method like in your question:

    mov edx, cr0
    mov ecx, 32          ; 32 bits in a dword
    mov ebx, 000B8000h
.loop:
    mov eax, 00000130h   ; BlueOnBlack "0"
    shl edx, 1           ; Top bit to the carry flag
    adc eax, 0           ; -> AL="0" or AL="1"
    mov [ebx], ax
    add ebx, 2
    dec ecx
    jnz .loop
halt:
    cli
    hlt
    jmp halt

Same thing but this time in hexadecimal representation. Again the same output method like in your question:

    mov edx, cr0
    mov ecx, 8           ; 8 nibbles (groups of 4 bits) in a dword
    mov ebx, 000B8000h
.loop:
    rol edx, 4
    mov eax, edx
    and eax, 15
    add eax, 00000130h
    cmp al, '9'          ; "0" to "9" are fine
    jbe .ok
    add eax, 7           ; This produces "A" to "F"
.ok:
    mov [ebx], ax
    add ebx, 2
    dec ecx
    jnz .loop
halt:
    cli
    hlt
    jmp halt

For a solution that uses a lookup table see:

How to convert a binary integer number to a hex string?

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Normaly you would write these conversions in a subroutine that you can call repeatedly for all sorts of numbers. However since this is bootloader code where perhaps you only need this one display, the current approach could be best (smallest codesize).

Answered By - Sep Roland