lesson 5 with hex printer

05-bootsector-functions-strings/boot_sect_print_hex.asm

@@ -0,0 +1,38 @@
+; receiving the data in 'dx'
+; For the examples we'll assume that we're called with dx=0x1234
+print_hex:
+    pusha
+
+    mov cx, 0 ; our index variable
+
+loop:
+    cmp cx, 4 ; loop 4 times
+    je end
+    
+    mov ax, dx ; we will work on ax
+    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'
+
+    ; 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
+    ror dx, 4 ; 0x1234 -> 0x4123 -> 0x3412 -> 0x2341 -> 0x1234
+
+    ; increment index and loop
+    add cx, 1
+    jmp loop
+
+end:
+    ; prepare the parameter and call the function
+    ; remember that print receives parameters in 'bx'
+    mov bx, HEX_OUT
+    call print
+
+    popa
+    ret
+
+HEX_OUT:
+    db '0x0000',0 ; reserve memory for our new string