Need help with Assembly language please

Modify the lab from the Fahrenheit to Celsius lab to only perform a conversion if the Fahrenheit value is above 0x20 C-style pseudocode to describe what needs to be done is given below:

if(Fahrenheit < 0x20) { print("ERROR: The Fahrenheit temperature is less than 0x20"); // print error message exit0; // exit the program }

Essentially the program should exit if the value that was input is less than 0x20. This program modification requires the following:

Create the error message in the .data segment

convert the pseudocode to assembly using the following hints:

compare the 1-byte Fahrenheit variable value to 0x20

use a conditional jump and jump if greater to the section of the program that does the Fahrenheit to Celsius conversion

if the conditional jump is not taken:

use print_string function to print the ERROR message

use the exit0 macro to exit the program

Here is a compilation option which will run the program twice, first with a valid input and next with an input that should cause the error message to be printed:

nasm -f elf *.asm; ld -m elf_i386 -s -o lab3 *.o; lab3 0x40; lab3 0x19

When the compilation option shown above is used, the following output should be produced:

Farhenheit Temperature:0x40

Celsius Temperature: = 0x11

Farhenheit Temperature:0x19

The Fahrenheit temperature is less than 0x20

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;;;;;;;;;;;;;;; Fahrenheit to Celcsius with valid input check;;;;;;;;;;;;;;;;;

; recieves an 8-bit HEX Fahrenheit Temperature through command line parameter

; outputs the Celsius approximation (in HEX) to the std_out

;

;

; Compiler option: nasm -f elf *.asm; ld -m elf_i386 -s -o lab3 *.o; lab3 0x40

;

; example output when input is valid:

; Fahrenheit Temperature: 0x40

; Celsius Temperature: 0x11

;

;

; example output when input is too low:

;

; Fahrenheit Temperature: 0x1F

;

; ERROR: The Fahrenheit temperature is less than 0x20

;

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;;;;;;;;;;;;;;;;;;;; CONSTANT DEFINITIONS ;;;;;;;;;;;;;;;;;;;;

null equ 0x00

MAXARGS equ 1

sys_exit equ 1

sys_read equ 3

sys_write equ 4

stdin equ 0

stdout equ 1

stderr equ 3

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;;;;;;;;;;;;;;;;;;;;;;; MACRO DEFINITIONS ;;;;;;;;;;;;;;;;;;;;;;;;

; print_char macro

; prints one ascii character to the console

%macro print_char 1

mov eax, 4 ;system call number (sys_write)

mov ebx, stdout ;file descriptor (stdout)

mov ecx, %1 ;address of data to print

mov edx, 1 ;number of bytes to print

int 0x80 ;do it!

%endmacro

; exit0 macro

; exits program with return code 0

%macro exit0 0

mov ebx, 0

mov eax, sys_exit

int 0x80

%endmacro

;;;;;;;;;;;;;;;;;;;; END MACRO DEFINITIONS ;;;;;;;;;;;;;;;;;;;;

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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;;;;;;;;;;;;;;;;;;;;;;;; DATA SEGMENT ;;;;;;;;;;;;;;;;;;;;;;;;

SECTION .data

hexprefix db "0x",null

debugOK db "OK",null

nl db 0x0a,0x0d

fmsg db "Farhenheit Temperature:",null

cmsg db "Celsius Temperature: = ",null

Fahrenheit db 0xff ; 0xff is default value

Celsius db 0xff ; 0xff is default value

;;;;; error messages ;;;;;

szErrMsg db "Too many arguments. The max number of args is !", null

szLineFeed db 10

szBErrMsg db "Invalid number of hex digits entered.",null

properMsg db "Proper 2 digit hex value: 0x4F",null

arg1nullMsg: db "First argument is null",null

arg2nullMsg: db "Second argument is null",null

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;;;;;;;;;;;;;;;;;;;; BSS SEGMENT ;;;;;;;;;;;;;;;;;;;;

section .bss

tmpbyte resb 1 ;hold byte temporarily for the hex to ascii conversion

tmphexchar resb 2 ;holds hex version of ascii char to be printed

arg1hex resb 1 ;holds the hex value of first argument

arg2hex resb 1 ;holds the hex value of second argument

arg1ascii resb 5 ;holds the ascii version of first argument (4 characters and a null)

arg2ascii resb 5 ;holds the ascii version of second argument (4 characters and a null)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;;;;;;;;;;;;;;;;;;;; TEXT SEGMENT ;;;;;;;;;;;;;;;;;;;;

SECTION .text

global _start

_start: ;;;;; BEGIN MAIN PROGRAM SECTION ;;;;;

nop

;;;;;;;;;;;;;;;;;;;; Get arguments from the stack ;;;;;;;;;;;;;;;;;;;;

push ebp ; save ebp on stack

mov ebp, esp ; set base pointer to stack pointer

cmp dword [ebp + 4], 1 ; check to see if no args were received (arg count will always be at least 1)

je NoArgs ; if no args entered (arge count == 1) then go to program exit section

cmp dword [ebp + 4], MAXARGS+1 ; check if total args entered is more than the maximum

ja TooManyArgs ; if total is greater than MAXARGS, show error message and exit

mov ebx, 3 ; ebx is index into the argument pointer array

; since ebp was pushed, args pointer array starts @ [ebp+12] = [ebp +4*3] = [ebp+4*ebx]

;;;;; Get first command line argument! ;;;;;

mov edi, dword [ebp + 4 * ebx] ; put pointer address of an arg into edi

test edi, edi ; test to see if pointer address is null (i.e. check if edi equal to 0)

jz arg1Null ; exit loop if edi == 0 (argument 1 is a null string)

call GetStrlen ; string length will be returned in edx register

mov ecx, dword [ebp + 4 * ebx] ; put address of first argument into ecx register

; EXIT program if invalid length hex value detected in first arg

cmp edx, 4 ; check for length of string to be 4 (not including null character)

jne invalidHexByte ; if the lengh is incorrect then go to show error and exit

; put first arg string into arg1ascii and then print to std_out

mov al, [ecx]

mov [arg1ascii], al ; put first character into arg1ascii

mov al, [ecx + 1]

mov [arg1ascii + 1], al ; put second character into arg1ascii

mov al, [ecx + 2]

mov [arg1ascii + 2], al ; put third character into arg1ascii

mov al, [ecx + 3]

mov [arg1ascii + 3], al ; put fourth character into arg1ascii

mov byte [arg1ascii + 4], null ; put null character at end of arg1ascii

mov edi, fmsg

call print_string

mov edi, arg1ascii ;output argument 1 in ascii

call print_string

call print_nl

;convert arg1ascii to raw data

mov eax, [arg1ascii] ; four ascii digits is the 32-bits that will go into eax to be converted

call ascii_hex_byte_to_raw ; convert the ascii hex quantity to raw data

mov [Fahrenheit], al

;;;;;;;;;;;;;;;;;;;;; DO NOT MODIFY ANYTHING ABOVE THIS COMMENT!

;;;;;;;;;;;;;;;;;;;;; perform celsius = 5 * (Fahrenheit-32) / 9

; check to see if the fahrenheit value is above 0x20

; if(Fahrenheit < 0x20)

; {

; print("The Fahrenheit temperature is less than 0x20");

; exit0

; }

;

Fahrenheit_greater_than_0x20:

;compute the Celsius value

mov al, [Fahrenheit] ; al = Fahrenheit value

sub al, 32 ; al = Fahrenheit - 32

mov dl, 5 ; prepare for multiplication

mul dl ; ax = 5 * (Fahrenheit-32)

mov dl, 9 ; prepare for division

div dl ; al = 5 * (Fahrenheit-32) / 9

mov [Celsius], al ;put sum of variable values into the "sum" variabel

;print Celsius message and the Celsius value

mov edi, cmsg ;put address of celsius message in edi register

call print_string ;print celsius message

mov edi, Celsius ;put address of Celsius variable in edi register

call print_hex_byte ;print Celsius HEX value

call print_nl ;print newline

;;;;;;;;;;;;;;;;;;;; DO NOT MODIFY ANYTHING BELOW THIS COMMENT!

exit0 ;Exit program to Operating System

NoArgs:

; No args entered,

; start program without args here

jmp arg1Null

TooManyArgs:

mov edi, szErrMsg

call print_string

call print_nl

exit0

invalidHexByte:

mov edi, szBErrMsg

call print_string

call print_nl

mov edi, properMsg

call print_string

call print_nl

exit0

arg1Null:

mov edi, arg1nullMsg

call print_string

call print_nl

exit0

arg2Null:

mov edi, arg2nullMsg

call print_string

call print_nl

exit0

;;;;; end of main function

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;;;;;;;;;;;;;;;;;;;; FUNCTIONS/SUBROUTINES ;;;;;;;;;;;;;;;;;;;;

;;print newline

; returns - nothing

print_nl:

mov eax, 4 ;system call number (sys_write) - p75 of Assembly Language Tutorial

mov ebx, 1 ;file descriptor (stdout)

mov ecx, nl ;address of data to print

mov edx, 2 ;number of bytes to print

int 0x80 ;do it!

ret

;;print hexadecimal prefix characters "0x"

; uses - eax, ebx, ecx, edx

; returns - nothing

print_0x:

mov eax, 4 ;system call number (sys_write) - p75 of Assembly Language Tutorial

mov ebx, 1 ;file descriptor (stdout)

mov ecx, hexprefix ;address of data to print

mov edx, 2 ;number of bytes to print

int 0x80 ;do it!

ret

;print_string

; recieves - address of a C-Style String in register edi

; C-Style means null terminated

; uses - eax, ebx, ecx, edx

; returns - nothing

print_string:

mov ecx, edi

checknull:

cmp byte [ecx],null

jz endstring

print_char ecx

inc ecx

jmp checknull

endstring:

ret

;print byte of data

; recieves - address of a data byte in register edi

; uses - eax, ebx, ecx, edx, tmpbyte, tmphexchar

; returns - nothing

print_hex_byte:

call print_0x ;print hex prefix

mov al, [edi] ;get hex byte to be printed

mov [tmpbyte], al ;put hex byte into tempbyte variable

and byte [tmpbyte], 0x0f ;isolate lower hex digit in bl

shr al, 4 ;isolate upper hex digit in al

;

mov ah, al ;pass upper hex digit to hex_char_to_ascii

call hex_char_to_ascii ;convert upper hex digit to ascii

mov [tmphexchar], ah

print_char tmphexchar

;

mov ah, [tmpbyte] ;pass lower hex digit to hex_char_to_ascii

call hex_char_to_ascii ;convert lower hex digit to ascii

mov [tmphexchar], ah

print_char tmphexchar

;

ret

;convert one hex character to ascii

; recieves - hex value in register ah

; uses - ah register

; returns - ascii encoded hex char in ah register

hex_char_to_ascii:

cmp ah, 10

jl hexlessthan10 ;check if hex digit is a through f

add ah, 0x07 ;adjust value for hex digit a through f

hexlessthan10:

add ah, 0x30 ;add ascii encoding

ret

;convert one hex byte in ascii to a raw data byte

; recieves - ascii encoded hex value with 0x prefix in register ax

; uses - ah register

; returns - raw data hex value in al register

ascii_hex_byte_to_raw:

ror eax, 16

cmp al, '9' ; see if hex character is '9' or less

jle lowCharLT9 ;

sub al, 7 ; offset the ascii value if it is A-F

lowCharLT9:

and al, 0x0f ; remove ascii encoding

cmp ah, '9' ; see if hex character is '9' or less

jle highCharLT9 ;

sub ah, 7 ; offset the ascii value if it is A-F

highCharLT9:

and ah, 0x0f ; remove ascii encoding

shl al, 4

add al, ah

ret

;get length of a C-Style string from command line argument passed in stack

;taken from http://www.dreamincode.net/forums/topic/285550-nasm-linux-getting-command-line-parameters/

; recieves - address of string in edi register

; uses - eax, ecx registers

; returns - address of the string into edx register

GetStrlen:

push ebx ;put ebx register data on the stack

xor ecx, ecx ;clear ecx register

not ecx ;set ecx = 0xffffffff, ecx will be decremented by repne as non-null characters are counted

xor eax, eax ;clear eax register, al will be used by scasb to search for null character

cld ;clear direction flag, index registers are incremented

repne scasb ;search for 0 in string; if not found edi++, ecx--, and check next character

;scasb: compares AL and [ES:EDI], EDI +=1 pcasm page 109

;repne: repeat instruction while string char not null (string char != AL) pcasm page 110

mov byte [edi - 1], 10 ;append newline character in place of null?

not ecx ;1's complement ecx, it will now contain address of an arg

pop ebx ;restore ebx value from stack

lea edx, [ecx - 1] ;put address of the string into edx register??

ret

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