Accidental infinite loop in Assembly

Question

So my assignment is to create V made out of asterisk(*) with each * having a random foreground and background color. Here is my code... I put a few breaks in and traced the program and somewhat figured out what the issue it. When I run it, it becomes an infinite loop because the backslash PROC calls the color Procedure which overwrites the loop counter(ECX register) and also overwrites the DH/DL registers used to move the cursors location. I am a beginner to assembly and could use a few hints or tips to avoid these problems in the future and fix it. Any help is appreciated, thanks in advance!

Assignment Guidelines - https://docs.google.com/document/d/1iPqfTd0qNOQo_xubVvsZLqfeNDog8mK6kzGGrR6s-OY/edit?usp=sharing

 ; main.asm - Assembly language source file
 ; Author:       Dekota Brown
 ; Date:             2/21/2017
 ; Description:  Colorful V-Pattern

 INCLUDE Irvine32.inc                   ; Irvine's assembly library
 ExitProcess PROTO,dwExitCode:DWORD     ; MS Windows ExitProcess function

.data
  nullVar DWORD ?
  msgEnd BYTE "Is the program running as you thought?",0
  msgEndCaption BYTE "Program Exit...",0
  symbol BYTE '*',0
.code
main PROC                               ; main procedure, entry point

mov EAX, nullVar
mov EBX, nullVar
mov ECX, nullVar
mov EDX, nullVar

call backslash

mov EDX,OFFSET msgEnd
mov EBX,OFFSET msgEndCaption
call MsgBoxAsk


mov EAX,07
call SetTextColor
call CrLf
call WaitMsg

INVOKE ExitProcess,0                ; end the program

main ENDP

color PROC

    call Randomize  ; Seed the RNG
    mov ECX,20  ; Set up loop counter
L1:
    mov EAX, 256
    call RandomRange
    call SetTextColor
    mov EDX,OFFSET symbol
    call WriteString
loop L1

    ret
color ENDP

backslash PROC

    mov dl, 2   ; Row 2
    mov dh, 4   ; Column 4
    mov ECX,20  ; Sets up loop counter
L2:
    call color
    call CrLf
    add dh,1    ; Increments column or shifts right by 1 position 
loop L2

    ret
backslash ENDP

forwardslash PROC

    ret
forwardslash ENDP

END

Answer 1

Good job on determining what the problem is. When faced with this problem (since there is only one ECX register), you need the color proc to save the previous value, use it, then restore the previous value. You can do this with the push and pop instructions:

color PROC
    push ecx ; ***** save previous value
    call Randomize  ; Seed the RNG
    mov ECX,20  ; Set up loop counter
L1:
    mov EAX, 256
    call RandomRange
    call SetTextColor
    mov EDX,OFFSET symbol
    call WriteString
loop L1
    pop ecx ; ***** restore previous value
    ret
color ENDP

I have marked the added code with *****.

For a given platform and operating system, there is something called an ABI which, among other things, states which registers are expected to be saved and restored by other functions you call. These are written down as rules that everybody follows so that code can be called between different compilers and languages without overwriting register values.