What are the best practicises to interpret an assembly program without knowing what it's suppos-CodePudding

I'm studying to pass an assembly exam where we are supposed to interpret an assembly source file provided without comments or anything, which is a task i find particularly difficult. My question is if in these cases, which i assume happen quite often in a professional environment, there are clues to look for to determine what the flow and the purpose of the program is. I am able to recognize loops through branching and jump instructions, but not much else apart from those, and i can't seem to find any proper source online. I'll post here an example program for reference, which is the current one i'm having a hard time with, in case anyone would want to point out something.

.data
  data_:  .byte  20, -40, -80
  pow_:  .alloc  804
  d:  .alloc  8

.global main

main:
 sub:  
   MOV  r0, #-100
   MOV  r2, #-1
   EOR  r1, r2, r0
   ADD  r1, r1, #1
   LDR  r2, =data_
   ADR  r9, d
   ADR  r8, pow_
   LDRB  r3, [r2]
   LDRB  r4, [r2, #1]
   LDRB  r5, [r2, #2]
 mpt:  
   MUL  r6, r0, r0
   MUL  r6, r3, r6
   MUL  r7, r0, r4
   ADD  r6, r5, r6
   ADD  r6, r7, r6
   STR  r6, [r8]
   CMP  r0, #-100
   BGT  aft 
 r:  
   STR  r6, [r9, #4]
   STR  r0, [r9]
 cyc:
   ADD  r8, r8, #4
   ADD  r0, r0, #1
   CMP  r0, r1
   BLE  mpt 
   MOV  r15, r14
 aft: 
   LDR  r2, [r9, #4]
   CMP  r6, r2
   BLT  r
   B  cyc

This code is ARM7, the .alloc instruction isn't a real instruction but it's there just to signify an allocation of n bits under the alias provided. From my understanding the program is trying to perform a loop of some sort while keeping an iteration counter, but i can't get my head around even why there is an EOR there, which converts to #-100 and #-1 to a #99

Any kind of suggestion is welcome

CodePudding user response：

You can translate each assembly instruction to some high level form, and reduce them to a simpler form. Have a look at the following example translating your mpt, and you'll get the basic idea.

 mpt:  
   MUL  r6, r0, r0
   MUL  r6, r3, r6
   MUL  r7, r0, r4
   ADD  r6, r5, r6
   ADD  r6, r7, r6
   STR  r6, [r8]
   CMP  r0, #-100
   BGT  aft

First, translate line by line.

r6 = r0 * r0
r6 = r6 * r3
r7 = r0 * r4
r6 = r6   r5
r6 = r6   r7
*r8 = r6
if (r0 > -100) goto aft

Then, make it simpler.

r7 = r0 * r4
r6 = r0 * r0 * r3   r5   r7
*r8 = r6
if (r0 > -100) goto aft

You'll get some readable code by applying the same procedure to the other parts of your code.