CODEWITHSUNDEEP
assemblyx86x86-16
rares.urdea
rares.urdea

Reputation: 660

8086 assembly - how to add 2 bytes whose sum will be larger than a byte

Ok so the question is simple. If i have 2 random bytes, say 150 (a[0]) and 215(b[0]) and i want to add them. obviously their sum won't fit in a byte so if i add them i will get an overflow. I've tried storing one of the bytes in al and doing a cbw, so that i would have the same quantity only represented on the word ax, and add the second byte to that, but there's something i'm failing to understand since it doesn't work. Here is a sample code: 

data segment
  a db 150,182,211
  b db 215,214,236
data ends

code segment
start:
  mov ax,data
  mov ds,ax

  lea si,a          ; these 2 shouldn't be here since i realised eventually that
                    ; i could use
  lea di,b          ; a constant for memory addressing and not necessarily a
                    ; a register
  mov ax,0000
  mov bl,a[0]
  mov al,b[0]
  cbw
  adc bx,ax      ; so this didn't work out well

  mov ax,0000
  mov al,a[0] 
  cbw            ; convert one of the bytes into a word
  mov cx,ax      ; and save it in cx
  mov al,b[0]    
  cbw            ; also convert the other byte into the word ax
  add ax,cx      ; add the two words
                 ; and that also failed

Upvotes: 2

Views: 8933

Answers (1)

Alexey Frunze
Alexey Frunze

Reputation: 62048

Say, you need to add two bytes, each having a value from 0 to 255 inclusive.

You need to add those bytes and save the value of the carry flag after the addition, which will be the 9th bit of the sum.

Here's how you could do it:

mov al, byte1
mov ah, 0 ; ax = byte1
add al, byte2
adc ah, 0 ; ax = byte1 + byte2

Note, I'm using mov ah, 0 instead of cbw when extending an 8-bit value to 16 bits. cbw works if your byte is supposed to represent negative values as well as positive, IOW, if it's in the range -128 to 127 instead of 0 to 255. And you're saying you've got 150 (0x96) and 215 (0xD7), so they have to be used as unsigned or non-negative values. If you apply cbw on them anyway, you'll get: -106 (0xFF96) and -41 (0xFFD7). And that's hardly your original numbers, right?

Upvotes: 7

Related Questions