# Assembly: Sum up the single bytes of a 32bit-register to a checksum

Exercise description:

"Write a program that takes a double word (4 bytes) as an argument, and then adds all the 4 bytes. It returns the sum as output. Note that all the bytes are considered to be of unsigned value.

Example: For the number 03ff0103 the program will calculate 0x03 + 0xff + 0x01 + 0x3 = 0x106, and the output will be 0x106

HINT: Use division to get to the values of the highest two bytes."

Full description on GitHub: xorpd

The code I've written:

format PE console
entry start

include 'win32a.inc'

; ===============================================

start:
mov     eax,    0x01020304

xor     ebp,    ebp
process_eax:
movzx   ebx,    al
movzx   ebx,    ah
cmp     ebp,    0x1
je      print_result
xor     edx,    edx
mov     ebx,    0xffff
div     ebx
mov     ebp,    0x1
jmp     process_eax
print_result:
mov     eax,    ecx
call    print_eax   ; Provided by the teacher. Prints eax to the console.

exitProgram:
; Exit the process:
push    0
call    [ExitProcess]

include 'training.inc'


I think it works. I've tried it with different values and the sums were correct.

Screenshot with the output of the code above (with 0x01020304 as the hardcoded value).

But it's surely not the most efficient way to solve the exercise.

• "HINT: Use division to get to the values of the highest two bytes." What? Why not shift? A simple loop in which you add eax & 0xFF to the sum and shift eax right by 8 would suffice, wouldn't it? – kyrill Apr 17 '17 at 15:02
• Comment your code. Regardless of language(, but in keeping with language conventions). What is mov ebx, 0xffff div ebx supposed to do? Does your code work for 0xdeadface? – greybeard Apr 17 '17 at 20:37
• @kyrill Thought about shifts too. I know it from Java. But because it will be topic of the NEXT lecture I thought that I'm expected to find a way without using it. – michael.zech Apr 18 '17 at 2:52
• @greybeard Yep. You are right. I admit I have meglected comments. Will take better care concerning it. – michael.zech Apr 18 '17 at 2:53

Since you're still learning, I won't cheat you out of the opportunity to discover for yourself, but I will offer some words of advice on how you can improve your program.

## Minimize register use

The current code uses eax, ebx, ecx, edx and ebp. One of the most important things for an assembly language programmer is to use registers efficiently and effectively. This particular task can easily be done with just two registers.

## Prefer shift to division

As alluded to in a comment, shift instructions are typically much faster to execute than divide instructions. For that reason, in tasks like this, it's much more common to see a shift than a divide.

## Avoid loops

Branching tends to be computationally disruptive for processors. While modern desktop machines tend to compensate for this via speculative execution and large cache sizes, code often runs faster if loops and branches are avoided entirely. This can confer other benefits such as more predictable running time which can be important for the scheduling of Real Time Operating Systems (RTOS) and in some kinds of cryptographic code to provide some resistance to side channel attacks.

## There are a few errors in this program.

• You build the result in ECX but you did not clear that one beforehand. If results are correct, as you stated, it's because the ECX register was empty and you got lucky.

• To bring the high word down to the low word, you need to divide by 65536, not by 65535 (0xffff) like you did.

## Optimizations.

• Instead of dividing a mere shift down by 16 would produce the same result.

Of course I noticed that the task hinted to use the division operation, but then again a hint is just a hint, not something mandatory!

• The second movzx ebx, ah could be written also as mov bl,ah since the highest 24 bits of EBX are still empty.

• You're using EBP as a flag (values 0 and 1 only). You can replace  cmp ebp, 0x1 by the shorter test ebp, ebp. Remember to jump on the opposite condition: jnz print_result.

• You're using EBP as a flag (values 0 and 1 only). You can replace  mov ebp, 0x1 by the shorter inc ebp.

## Your program but modified based on the above.

start:
mov     eax, 0x01020304
xor     ecx, ecx
xor     ebp, ebp
process_eax:
movzx   ebx, al
mov     bl, ah
test    ebp, ebp
jnz     print_result
xor     edx, edx
mov     ebx, 0x10000
div     ebx
inc     ebp
jmp     process_eax
print_result:


## Your program but modified more using 1 register less.

Only repeat the code when the quotient produced a non-zero AX.

start:
mov     eax, 0x01020304
xor     ecx, ecx
process_eax:
movzx   ebx, al
mov     bl,  ah
xor     edx, edx
mov     ebx, 0x10000
div     ebx
test    ax, ax
jnz     process_eax     ;At most 1 time


## Your program but modified more using 2 registers less and preferring to use shift over divide.

Only repeat the code when the quotient produced a non-zero AX.

start:
mov     eax, 0x01020304
xor     ecx, ecx
process_eax:
movzx   ebx, al