atbash.asm

; File: toupper.asm last updated 09/17/2018
;
; Convert user input to upper case.
;
; Assemble using NASM:  nasm -f elf64 toupper.asm
; Link with ld:  ld toupper.o -o toupper
;

%define STDIN         0
%define STDOUT        1
%define SYSCALL_EXIT  60
%define SYSCALL_READ  0
%define SYSCALL_WRITE 1
%define BUFLEN        256


        SECTION .data                ; initialized data section

msg1:   db "Enter string: "          ; user prompt
len1:   equ $-msg1                   ; length of first message

msg2:   db "Original:  "             ; original string label
len2:   equ $-msg2                   ; length of second message

msg3:   db "Converted: "             ; converted string label
len3:   equ $-msg3

msg4:   db 10, "Read error", 10      ; error message
len4:   equ $-msg4


        SECTION .bss                 ; uninitialized data section
buf:    resb BUFLEN                  ; buffer for read
newstr: resb BUFLEN                  ; converted string
count:  resb 4                       ; reserve storage for user input bytes


        SECTION .text                ; Code section.
        global  _start               ; let loader see entry point

_start: nop                          ; Entry point.
start:                               ; address for gdb

        ; prompt user for input
        ;
        mov rax, SYSCALL_WRITE       ; write function
        mov rdi, STDOUT              ; Arg1: file descriptor
        mov rsi, msg1                ; Arg2: addr of message
        mov rdx, len1                ; Arg3: length of message
        syscall                      ; 64-bit system call

        ; read user input
        ;
        mov rax, SYSCALL_READ        ; read function
        mov rdi, STDIN               ; Arg1: file descriptor
        mov rsi, buf                 ; Arg2: addr of message
        mov rdx, BUFLEN              ; Arg3: length of message
        syscall                      ; 64-bit system call

        ; error check
        ;
        mov [count], rax             ; save length of string read
        cmp rax, 0                   ; check if any chars read
        jg  read_OK                  ; >0 chars read = OK
        mov rax, SYSCALL_WRITE       ; Or Print Error Message
        mov rdi, STDOUT              ; Arg1: file descriptor
        mov rsi, msg4                ; Arg2: addr of message
        mov rdx, len4                ; Arg3: length of message
        syscall                      ; 64-bit system call
        jmp     exit                 ; skip over rest
read_OK:


        ; Loop for upper case conversion
        ; assuming count > 0
        ;
L1_init:
        mov     rcx, [count]         ; initialize count
        mov     rsi, buf             ; point to start of buffer
        mov     rdi, newstr          ; point to start of new string

L1_top:
        mov     al, [rsi]            ; get a character
        inc     rsi                  ; update source pointer
        cmp     al, 'A'              ; less than 'A'?
        jb      L1_cont		     ; skip to next character
        cmp     al, 'Z'              ; less than 'Z'?
        jbe     L1_convd	     ;convert with upper case script
	cmp	al, 'a'		; less than 'a'
	jb	L1_cont		; skip to next character
	cmp	al, 'z'		;less than 'z'
	jbe	L1_convc	;convert with lower case script
	jmp	L1_cont		;jump to next character

L1_convc:
	mov	bh,'m'		;put 'm' into bh
	sub	bh,al		;find distance of 'm' to al in ascii
	neg	bh		;find negative of that distance
	sub	al,bh		;subtract value from al
	sub	al,bh		;subtract vaue again
	add	al,1		;add one (necessary for algorithm)
	jmp	L1_cont		;jump to next character

L1_convd:
        mov     bh,'M'		;put 'M' into bh
        sub     bh,al		;find distance of 'M' to al in ascii
        neg     bh		;find negative of that distance
        sub     al,bh		;subtract value from al
        sub     al,bh		;subtract value agin
        add     al,1		;add one
	jmp	L1_cont		;jump to next character
	
	
L1_cont:
        mov     [rdi], al            ; store char in new string
        inc     rdi                  ; update dest pointer
        dec     rcx                  ; update char count
        jnz     L1_top               ; loop to top if more chars
L1_end:


        ; print out user input for feedback
        ;
        mov rax, SYSCALL_WRITE       ; Print Message
        mov rdi, STDOUT              ; Arg1: file descriptor
        mov rsi, msg2                ; Arg2: addr of message
        mov rdx, len2                ; Arg3: length of message
        syscall                      ; 64-bit system call

        mov rax, SYSCALL_WRITE       ; Write user input
        mov rdi, STDOUT              ; Arg1: file descriptor
        mov rsi, buf                 ; Arg2: addr of message
        mov rdx, [count]             ; Arg3: length of message
        syscall                      ; 64-bit system call

        ; print out converted string
        ;
        mov rax, SYSCALL_WRITE       ; Print Message
        mov rdi, STDOUT              ; Arg1: file descriptor
        mov rsi, msg3                ; Arg2: addr of message
        mov rdx, len3                ; Arg3: length of message
        syscall                      ; 64-bit system call


        mov rax, SYSCALL_WRITE       ; Write out string
        mov rdi, STDOUT              ; Arg1: file descriptor
        mov rsi, newstr              ; Arg2: addr of message
        mov rdx, [count]             ; Arg3: length of message
        syscall                      ; 64-bit system call


        ; final exit
        ;
exit:   mov rax, SYSCALL_EXIT
        mov rdi, 0                   ; exit to shell
        syscall