How do I use ReadString in Assembly?

Question

mov  edx,offset Prompt1     
call WriteString

mov  ecx,32     
mov  edx,offset String1     
call ReadString

Now, how do I go about accessing String1? How do I move it into a register so I can do shifting operations on it?

For example,

mov eax, edx
shr eax, 1

The problem that I'm having is that I can't figure out how to access String1. It doesn't seem to be going into the eax register, but if I call WriteString it will appear, so I believe it is in EDX.

Answer 1

The data is read into memory starting at the address of String1. There is a null byte (0) after the last input character.

So if after your call ReadString, you write mov edx,offset String1, then EDX is pointing to the first character of the string.

You can then process the string. For example, to add 1 to each character:

    call ReadString
    mov edx,offset String1
theLoop:
    mov al,[edx]
    cmp al,0
    jz done    ; the character is 0, so done
    inc al
    mov [edx],al
    inc edx    ; next character
    jmp theLoop
done:

So if the input was "0123abc", that would change the string to "1234bdc".

(for the nitpickers: Yes, I know this could be optimized. In particular the cmp al,0. But it's better for beginners to think of comparing ... we can work on optimizations after they have a firm grasp of working at the processor level.)

Answer 2

ReadString requires the buffer address and the size placed in EDX and ECX respectively. So you have to ensure both registers have the necessary information before referring to it. You can do this by storing those values in your data sections, although the size usually only needs to be an assemble-time constant, not actually stored in memory in .data.

So before calling ReadString, you have to set up both args

    MOV     EDX, OFFSET String1     ; pointer
    MOV     ECX, SIZEOF String1 - 1 ; max byte count, not including terminating 0
    call    ReadString
 ; EAX return value = number of bytes actually read
 ; no other registers modified in Irvine32's unusual calling convention

Online docs for Irvine32:
https://csc.csudh.edu/mmccullough/asm/help/source/irvinelib/readstring.htm

Reads a string of up to ECX non-null characters from standard input, stopping when the user presses the Enter key.
A null byte is stored following the characters input, but the trailing carriage return and line feed characters are not placed into the buffer.
ECX should always be smaller than the buffer size (never equal to the buffer size) because the null byte could be the (ECX+1)th character stored.

The docs also have a similar example to this, showing reserving a buffer as well.

Answer 3

INCLUDE Irvine32.inc

.data


buffer BYTE 21 DUP(0)
byteCount DWORD ? 


.code
main PROC

mov   edx, OFFSET buffer
mov   ecx, SIZEOF buffer
call  ReadString
mov   byteCount, eax


call writestring


exit
main ENDP
END main