x86 32-bit Assembly Practice using NASM assembler

Implementation Overview

• Used 32-bit registers (eax, ebx, ecx, etc.)
• Utilized helper variables to free up registers
• Implemented the columnar function on the second day
• Encountered obstacles in traversing the matrix formed from the input string
• Spent hours debugging using valgrind and GDB

Variables and Fields

ages.asm

• day: Day from the date of birth
• month: Month from the date of birth
• year: Year from the date of birth
• aux: Helper variable to free up eax register, stores the value of 'len'

columnar.asm

• aux_len: Stores the number of rows
• aux_helper: Preserves the address in edx
• copy_len_key: Saves the value stored by len_cheie
• contor: Counter used to construct 'ciphertext'

cache.asm

• aux_reg: Stores the register address
• aux_address: Stores the address given as an argument
• aux_offset: Helps calculate the offset
• contor: Used in loops and offset calculation
• aux_helper_offset: Stores the offset value (3 bits)
• aux_helper: Preserves important values for matrix position determination

Implementation Details

rotp function

• Constructs 'ciphertext' string in 'create_string' loop using 'loop' instruction
• Uses 'bl' subregister for XOR operation (1 byte per character)
• Increments esi address to get next character
• Stores result in edx register, operating with edx+eax

ages function

• Handles all cases to calculate ages
• Iterates through 'my_date' vector using eax register
• Uses construction [vector_name+type_size*index+struct.year] to retrieve year
• Implements logic for different date scenarios (e.g., birth date after present date)

columnar function

• Transforms string into matrix form
• Main idea illustrated with examples:

esi->  Hai s|a dam| mana| cu m|ana! (test 1)
esi->  Nu te| intr|eba n|imeni| nimi|c! (test 2)

   0 1 2 3 4
 1 h a i   s
 2 a   d a m      <- Test 1
 3   m a n a
 4   c u   m
 5 a n a ! -     
        
   0 1 2 3 4                  
 1 N u   t e                          
 2   i n t r                           
 3 e b a   n      <- Test 2                
 4 i m e n i                       
 5   n i m i                       
 6 c ! - - -

• Uses construction [esi+edx*(eax-1)+ecx] to traverse string correctly
• Calculates number of rows and columns
• Constructs 'ciphertext' string in loops
• Implements logic to avoid invalid reads

cache function

• Most complex implementation
• Uses global variables to free up registers
• Calculates offset and tag of address in edx
• Extracts last 3 bits from address
• Transforms row and column into single index
• Implements 'search_tag' loop to find previously calculated tag
• Handles 'cache miss' scenario by placing elements on 'to_replace' line
• Directly puts value from edi into register for 'tag_found' case

Matrix Indexing Example

Let the matrix be of size 4x3
This matrix is stored in esi

 1  4  5
 6  7  4
44 67 43
34 76 45

In memory: 1 4 5 6 7 4 44 67 43 34 76 45

To access element 34, use: [esi+3*3+0]

The construction to access an element k on row i and column j is: [esi+i*size_column+j]