Program to Shift String By N Bits In Assembly Language Assignment Solution

Instructions

Objective

Write an Assembly language assignment program to shift string by n bits in.

Requirements and Specifications

You are responsible for writing an assembly function for an encryption library. This function will encrypt or decrypt a string of characters. It will use a simple ciphertext algorithm which does character shifting. This function will use the C function signature but everything within this function should be assembly code using the ASM block similar to the assembly example shown in class. This function should be built into a separate shared library (e.g. .DLL or .so). p3.cpp will contain the calling function and should not be modified.

Program Specification:

long s_encrypt (char *src, char* dest, long en_flag);

src – the original string

dest – the output string

en_flag – 0 for encryption and 1 for decryption

This function returns the # of characters being changed in the string. If there is no change, it should return zero.

Screenshots of output

Source Code

/* encrypt.cpp - This file contains the code to encrypt and decrypt an input string and output the modified string. */ #include "key.h" #include long s_encrypt (char *src, char* dest, long en_flag); void get_key (long *dir, long *count); void set_key (long dir, long count); void get_key (long *dir, long *count) { *dir = direction; *count = shiftcount; } void set_key (long dir, long count) { direction = dir; shiftcount = count; } long s_encrypt (char *src, char* dest, long en_flag) { long characters_changed = 0; long len = strlen(src); long count = shiftcount % 26; /* adjust shiftcount if > 26 */ long dir = direction; asm ( /* Add your ASM code here !! */ "movq %1, %%rsi;" // load source pointer "movq %2, %%rdi;" // load destination pointer "movq %3, %%r8;" // load flag "movq %4, %%rcx;" // load string length "movq $0, %%rax;" // set changed count to zero "cmpq $0, %%rcx;" // if empty string "je done;" // go to end "movq %5, %%rdx;" // load shift count "movq %6, %%r9;" // load direction "addq $1, %%r9;" // increment direction to get 1 or 2 "cmpq $1, %%r9;" // if forward (1) "je loop_start;" // then start "movq $-1, %%r9;" // else set negative increment "addq %%rcx, %%rsi;" // point to end of string "decq %%rsi;" // point to last char in string "loop_start:" "movb (%%rsi), %%r10b;" // load character from source string "cmpb $65, %%r10b;" // compare char with 'A' "jl save;" // if < 'A' is not alpha, leave as it is "cmpb $90, %%r10b;" // compare char with 'Z' "jg checkLow;" // if > 'Z' is not upper case, might be lower "addb $32, %%r10b;" // else, toggle to lower case "movb $122, %%bl;" // save 'z' for comparison "cmpq $0, %%r8;" // see if we need to encrypt "je encrypt;" // if so, encrypt "movb $97, %%bl;" // else, save 'a' for comparison "jmp decrypt;" // jump to decryption part "checkLow:" "cmpb $97, %%r10b;" // compare char with 'a' "jl save;" // if < 'a' is not alpha, leave as it is "cmpb $122, %%r10b;" // compare char with 'z' "jg save;" // if > 'z' is not alpha, leave as it is "subb $32, %%r10b;" // else, toggle to upper case "movb $65, %%bl;" // else, save 'A' for comparison "cmpq $1, %%r8;" // see if we need to decrypt "je decrypt;" // if so, encrypt "movb $90, %%bl;" // save 'Z' for comparison "encrypt:" "incq %%rax;" // increment changed chars "addb %%dl, %%r10b;" // shift by required amount "cmpb %%bl,%%r10b;" // compare with 'z' or 'Z' "jle save;" // if <= 'z' or 'Z', save "subb $26,%%r10b;" // else, wrap around "jmp save;" // save encrypted char "decrypt:" "incq %%rax;" // increment changed chars "subb %%dl, %%r10b;" // shift by required amount "cmpb %%bl,%%r10b;" // compare with 'a' or 'A' "jge save;" // if >= 'a' or 'A', save "addb $26,%%r10b;" // else, wrap around "save:" "movb %%r10b, (%%rdi);" "add %%r9, %%rsi;" // advance to next char in source string "inc %%rdi;" // advance to next char in destination string "loop loop_start;" // repeat for all chars in source string "done:" "movb $0, (%%rdi);" // save ending zero in destination : "=a" (characters_changed) : "m" (src), "m" (dest), "m" (en_flag), "m" (len), "m" (count), "m" (dir) : ); return characters_changed; }