11 印出 hex

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寫一個可以印出 hex 內容的 function。

shr 是 bitwise 的 right shift instruction。右移的最後一個 bit 會放到 carry flag 裡。可以用 jc 跟 jnc 來做 branch，jc 是如果 carry flag 有被 set 則 jump，後者是沒 set 的情況會 jump。

第一版：處理最後一個字

這個版本可以印出 0x0006。基本概念是用 right shift 得知 dx 最右邊的 bit 是 0 還是 1，以 4 個 bit 為一單位處理。

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[org 0x7c00]

mov dx, 0x1fb6
call print_hex
jmp $

; prints the value of DX as hex.
print_hex:
    pusha

    ; only bx can be used as an index register
    mov bx, HEX_OUT
    add bx, 5                ; bx = HEX_OUT + 5

    shr dx, 1                ; dx >> 1
    jnc end_if_1
        mov cx, 1            ; if cf == 1
        add byte [bx], cl    ; [bx] += 1
end_if_1:
    
    shr dx, 1   ; dx >> 1
    jnc end_if_2
        mov cx, 2
        add byte [bx], cl    ; [bx] += 2
end_if_2:

    shr dx, 1   ; dx >> 1
    jnc end_if_3
        mov cx, 4
        add byte [bx], cl
end_if_3:

    shr dx, 1   ; dx >> 1
    jnc end_if_4
        mov cx, 8
        add byte [bx], cl
end_if_4:


    mov bx, HEX_OUT
    call print_string

    popa
    ret

%include "print_string.asm"

; global variables
HEX_OUT: db '0x0000', 0

; padding and magic number
times 510-($-$$) db 0
dw 0xaa55

然後遇到超過 10 就會錯…因為 ASCII code 在 '9' 後面不是 'a'。

第二版

修正 10 以上的錯誤

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[org 0x7c00]

mov dx, 0x1fb6
call print_hex
jmp $

; prints the value of DX as hex.
print_hex:
    pusha

    mov ax, 5
    ; only bx can be used as an index register
    mov bx, HEX_OUT
    add bx, ax               ; bx = HEX_OUT + ax

    mov cx, 0
    shr dx, 1                ; dx >> 1
    jnc end_if_1
        add cx, 1            ; if cf == 1
end_if_1:
    
    shr dx, 1  
    jnc end_if_2
        add cx, 2
end_if_2:

    shr dx, 1 
    jnc end_if_3
        add cx, 4
end_if_3:

    shr dx, 1
    jnc end_if_4
        add cx, 8
end_if_4:

    cmp cx, 10
    jl less_ten
        mov byte [bx], 'a'
        sub cx, 10
less_ten:
    add [bx], cx

    mov bx, HEX_OUT
    call print_string

    popa
    ret

%include "print_string.asm"

; global variables
HEX_OUT: db '0x0000', 0

; padding and magic number
times 510-($-$$) db 0
dw 0xaa55

第三版：加上外層 loop

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[org 0x7c00]

mov dx, 0x1fb6
call print_hex
jmp $

; prints the value of DX as hex.
print_hex:
    pusha

    mov ax, 5
loop:                           ; for (ax = 5; ax >= 2; ax--)
    cmp ax, 2
    jl end_loop
        mov cx, 0
        shr dx, 1               ; dx >> 1
        jnc end_if_1
            add cx, 1           ; if cf == 1, cx += 1
    end_if_1:
        
        shr dx, 1               ; dx >> 1
        jnc end_if_2 
            add cx, 2           ; if cf == 1, cx += 2
    end_if_2:

        shr dx, 1               ; dx >> 1
        jnc end_if_3         
            add cx, 4           ; if cf == 1, cx += 4
    end_if_3:

        shr dx, 1               ; dx >> 1
        jnc end_if_4
            add cx, 8           ; if cf == 1, cx += 8
    end_if_4:

		; only bx can be used as an index register
        mov bx, HEX_OUT
        add bx, ax              ; bx = HEX_OUT + ax

        cmp cx, 10
        jl less_ten
            mov byte [bx], 'a'  ; [bx] = 'a'
            sub cx, 10          ; cx -= 10
    less_ten:
        add [bx], cx            ; [bx] += cx

    sub ax, 1                   ; ax--
    jmp loop
end_loop:

    mov bx, HEX_OUT
    call print_string

    popa
    ret

%include "print_string.asm"

; global variables
HEX_OUT: db '0x0000', 0

; padding and magic number
times 510-($-$$) db 0
dw 0xaa55

第四版：拆成多個檔案

print_string.asm：

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print_string:           ; function name
    pusha               ; push all register to stack to preserve them

    mov ah, 0x0e        ; tty mode

loop_start:
    cmp byte [bx], 0    ; compare [bx] which is one byte to zero, for null terminating char
    je loop_end         ; if [bx] == 0, end loop
    mov al, [bx]        ; move char printed to al
    int 0x10            ; call print interrupt
    add bx, 1           ; bx + 1, move to next char
    jmp loop_start      ; run loop body again

loop_end:    
    popa                ; restore all register
    ret                 ; return to callee

print_hex.asm：

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; prints the value of DX as hex.
print_hex:
    pusha

    mov ax, 5
loop:                           ; for (ax = 5; ax >= 2; ax--)
    cmp ax, 2
    jl end_loop
        mov cx, 0
        shr dx, 1               ; dx >> 1
        jnc end_if_1
            add cx, 1           ; if cf == 1, cx += 1
    end_if_1:
        
        shr dx, 1               ; dx >> 1
        jnc end_if_2 
            add cx, 2           ; if cf == 1, cx += 2
    end_if_2:

        shr dx, 1               ; dx >> 1
        jnc end_if_3         
            add cx, 4           ; if cf == 1, cx += 4
    end_if_3:

        shr dx, 1               ; dx >> 1
        jnc end_if_4
            add cx, 8           ; if cf == 1, cx += 8
    end_if_4:

		; only bx can be used as an index register
        mov bx, HEX_OUT
        add bx, ax              ; bx = HEX_OUT + ax

        cmp cx, 10
        jl less_ten
            mov byte [bx], 'a'  ; [bx] = 'a'
            sub cx, 10          ; cx -= 10
    less_ten:
        add [bx], cx            ; [bx] += cx

    sub ax, 1                   ; ax--
    jmp loop
end_loop:

    mov bx, HEX_OUT
    call print_string

    popa
    ret

%include "print_string.asm"

boot_sect.asm：

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[org 0x7c00]

mov dx, 0x1fb6
call print_hex
jmp $

%include "print_hex.asm"

; global variables
HEX_OUT: db '0x0000', 0

; padding and magic number
times 510-($-$$) db 0
dw 0xaa55