fixed bug in print_hex

cfenollosa · cfenollosa · commit 04625d9a1f39 · 2014-10-08T18:25:10.000+02:00
diff --git a/05-bootsector-functions-strings/boot_sect_main.asm b/05-bootsector-functions-strings/boot_sect_main.asm
@@ -11,7 +11,7 @@ call print
 
 call print_nl
 
-mov dx, 0x1234
+mov dx, 0x12fe
 call print_hex
 
 ; that's it! we can hang now
diff --git a/05-bootsector-functions-strings/boot_sect_print_hex.asm b/05-bootsector-functions-strings/boot_sect_print_hex.asm
@@ -6,7 +6,8 @@ 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.
+; Numeric ASCII values: '0' (ASCII 0x30) to '9' (0x39), so just add 0x30 to byte N.
+; For alphabetic characters A-F: 'A' (ASCII 0x41) to 'F' (0x46) we'll add 0x40
 ; Then, move the ASCII byte to the correct position on the resulting string
 loop:
     cmp cx, 4 ; loop 4 times
@@ -15,13 +16,17 @@ loop:
     ; 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
-    add ax, 0x30 ; add 0x30 to N to convert it to ASCII "N"
+    add al, 0x30 ; add 0x30 to N to convert it to ASCII "N"
+    cmp al, 0x39 ; if > 9, add extra 8 to represent 'A' to 'F'
+    jle step2
+    add al, 7 ; 'A' is ASCII 65 instead of 58, so 65-58=7
 
+step2:
     ; 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'
+    mov [bx], al ; copy the ASCII char on 'al' to the position pointed by 'bx'
     ror dx, 4 ; 0x1234 -> 0x4123 -> 0x3412 -> 0x2341 -> 0x1234
 
     ; increment index and loop
