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lesson 5 with hex printer

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Carlos Fenollosa 2014-10-05 20:19:10 +02:00
parent 5346a84b36
commit 9bc3cd33e6
1 changed files with 11 additions and 8 deletions
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19
05-bootsector-functions-strings/boot_sect_print_hex.asm
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@ -5,20 +5,23 @@ print_hex:
mov cx, 0 ; our index variable
; Strategy: get the last char of 'dx', then convert to ASCII
; ASCII values: '0' (ASCII 0x30) to '9' (0x39), so just add 0x30 to byte N.
; Then, move the ASCII byte to the correct position on the resulting string
loop:
cmp cx, 4 ; loop 4 times
je end
mov ax, dx ; we will work on ax
; 1. convert last char of 'dx' to ascii
mov ax, dx ; we will use 'ax' as our working register
and ax, 0x000f ; 0x1234 -> 0x0004 by masking first three to zeros
; convert each hex value to its ASCII value. '0' (ASCII 0x30) to '9' (0x39)
; We will need to convert byte N to byte 0x3N by just adding 0x30
add ax, 0x30 ; 0x0004 -> ASCII '4'
add ax, 0x30 ; add 0x30 to N to convert it to ASCII "N"
; bx <- address of the character to replace = base + string length - index
mov bx, HEX_OUT + 5 ; last char of string, beware not to replace null char
sub bx, cx ; address of char to replace
or [bx], ax ; copy the ASCII char to the string position
; 2. get the correct position of the string to place our ASCII char
; bx <- base address + string length - index of char
mov bx, HEX_OUT + 5 ; base + length
sub bx, cx ; our index variable
or [bx], ax ; copy the ASCII char on 'ax' to the position pointed by 'bx'
ror dx, 4 ; 0x1234 -> 0x4123 -> 0x3412 -> 0x2341 -> 0x1234
; increment index and loop