What does the syntax mean for declaring an array, and for calling functions?

Question

For my first question, let's assume we have the following line of code under .data:
```
"theSINTArray BYTE 256 dup(?)". 
```
I know that this code creates an array in which each element must be a BYTE, but what are the 256 and dup(?) there for?
I know that the code below pushes the type, length and offset/address of theSINTArray onto the stack, but what I would like to know is if it is possible to retrieve them from the stack and utilize them within a subroutine.
```
main PROC

push    TYPE theSINTArray
push    LENGTHOF theSINTArray
push    OFFSET theSINTArray
call    testParameters
exit

main ENDP
```
This is a bit of a tedious question, so I apologize in advance, but I simply don't understand why a large portion of the lines in the code sample below are necessary. Assuming I have the line "prompt BYTE "Please enter a value: ",0" in the .data section, what is the purpose of each line of the code below? Note: WriteString and ReadString are subroutines defined in Irvine's library, which I am using.
```
testOutput PROC

push    edx
push    ecx
mov     edx,offset prompt
call    WriteString
pop     ecx
pop     edx
call    ReadString
ret

testOutput ENDP
```

score 4 · Accepted Answer · answered Apr 04 '16 at 19:30

what are the 256 and dup(?) there for?

Read the assembler's manual. TL;DR: reserves 256 uninitialized bytes

if it is possible to retrieve them from the stack and utilize them within a subroutine

Of course it is possible, argument passing would be silly if the callee couldn't access the arguments ;) You address them relative to esp (the stack pointer), or, after you have set it up as frame pointer, ebp. Examples: [esp+4] or [ebp+8].

what is the purpose of each line of the code below

testOutput PROC            ; begin testOutput procedure

push    edx                ; save edx on stack
push    ecx                ; save ecx on stack
mov     edx,offset prompt  ; load edx with address of prompt
                           ; presumably argument to WriteString
call    WriteString        ; invoke WriteString procedure
pop     ecx                ; restore ecx saved above
                           ; in case WriteString modified it
pop     edx                ; restore edx saved above (we have modified it)
call    ReadString         ; invoke ReadString procedure
ret                        ; return from subroutine

testOutput ENDP            ; end of procedure

Jester, your response definitely improved my understanding of the code samples, but I do have 2 remaining questions. For the first code example, when you say "reserve", do you mean set the number of indexes in the array? So "256 dup(?)" makes it so that theSINTArray has 256 indexes? Also, what would happen if the question mark next to dup was replaced with a different character such as a number? — Proto, Apr 04 '16 at 19:43
In other words, it allocates a 256 byte array, not initialized to any particular value you care about. If you do care, you can of course give that value instead of the `?`. — Jester, Apr 04 '16 at 19:48
Oh, now I understand the value within the parenthesis is used to initialize the value that is stored in each byte. Thank you for your help, Jester. — Proto, Apr 04 '16 at 19:51

What does the syntax mean for declaring an array, and for calling functions?

1 Answers1