OFFSET can also be applied to a direct-offset operand?

[RedSkeleton007]

My book speaks of something called a direct-offset operand. Here's the example it provides:

Code Select Expand


.data
myArray WORD 1,2,3,4,5
.code
mov esi,OFFSET myArray + 4

It then says that ESI points to the third integer in the array. Since array indexes start at

• :
  
• ,[1],[2],[3],[4]
  1,  2,   3,   4,  5
  wouldn't myArray + 4 point to the fourth integer at index [3]? What's going on here?

[hutch--]

Zero based index.

[RedSkeleton007]

Zero based index.

Okay... so then myArray + 4 does point to the integer in index [3], right?

[dedndave]

OFFSET myArray+0
would point to the first element

OFFSET myArray+4
would point to the last element

[hutch--]

Also remember the data SIZE, 1 byte for BYTE, 2 bytes for WORD, 4 bytes for DWORD etc .... OFFSET works in BYTES so you must set the offset to match the data type, 0,4,8,12,16 etc .... for DWORD.
0,2,4,6,8,10,12 etc .... for WORD.

[dedndave]

my mistake, Hutch - thanks for catching that

OFFSET myArray+4 does point to the 3 value   :P

[Tedd]

My book speaks of something called a direct-offset operand. Here's the example it provides:

Code Select Expand


.data
myArray WORD 1,2,3,4,5
.code
mov esi,OFFSET myArray + 4

It then says that ESI points to the third integer in the array. Since array indexes start at

• :
  
• ,[1],[2],[3],[4]
  1,  2,   3,   4,  5
  wouldn't myArray + 4 point to the fourth integer at index [3]? What's going on here?
  

"OFFSET myArray + 4" is (OFFSET myArray) + 4 bytes (the cpu knows nothing of your arrays or their element sizes.)
If you want array indexing, you must multiply the index by the element size, in this case: myArray = (OFFSET myArray) + (SIZEOF WORD*i)
So, +0 points to "1", +2 points to "2", +4 points to "3", +6 points to "4" and +8 points to "5".

[jj2007]

Code Select Expand

mov ax, myArray[2*WORD] ; simple

mov esi, offset myArray ; offset in register
mov dx, [esi+2*WORD]

mov ecx, 2
mov bx, [esi+ecx*WORD] ; using an index register

mov ax, myArray[ecx*WORD] ; also valid