What does OFFSET in 16 bit assembly code mean?

Question

I am going through some example assembly code for 16-bit real mode.

I've come across the lines:

    mov    bx, cs
    mov    ds, bx
    mov    si, OFFSET value1
    pop    es
    mov    di, OFFSET value2

what is this doing? What does having 'OFFSET' there do?

Belinda

Answer 1

As some of the other answers say, the offset keyword refers to the offset from the segment in which it is defined. Note, however, that segments may overlap and the offset in one segment may be different in another segment. For instance, suppose you have the following segment in real mode

data SEGMENT USE16 ;# at segment 0200h, linear address 2000h

    org 0100h
    foo db 0

    org 01100h
    bar db 0

data ENDS

The assembler sees that foo is at offset 0100h from the base of data SEGMENT, so wherever it sees offset foo it will put the value 0100h, regardless of the value of DS at the time.

For example, if we change DS to something other than the base of the data segment the assembler is assuming:

mov ax, 200h            ; in some assemblers you can use @data for the seg base
mov ds, ax

mov bx, offset foo          ; bx = 0100h
mov byte ptr [bx], 10       ; foo = 10


mov ax, 300h
mov ds, ax

mov bx, offset foo          ; bx = 0100h
mov byte ptr [bx], 10       ; bar = 10, not foo, because DS doesn't match what we told the assembler

In the second example DS is 0300h, so the base of the segment pointed to by DS is 03000h. This means that ds:[offset foo] points to the address 03000h + 0100h which is the same as 02000h + 01100h, which points to bar.

Freehold

Where does the 2000h come from? You know based on some other not-shown stuff that it's the seg base? Normally you'd use @data instead of a numeric literal, right? (This works well for an example, though.) – Tetragon

@PeterCordes Segment 2000h should be 200h, (same problem with 3000h) if it wants to match the description. The segment value 200 would have been arbitrarily chosen as an example with an absolute value for clarity (rather than @data) – Spandex

@MichaelPetch: good catch. I put back the 2000h linear address in the comment, too, as a reminder that segment register values are used <<4 = left by 1 hex digit. – Tetragon

Answer 2

It just means the address of that symbol. It's a bit like the & operator in C, if you are familiar with that.

Paramo

Answer 3

offset means that si register will be equal to the offset of the variable value1 (not to its actual value). Offset is the address from the beginning of memory segment where the variable is stored. The offset is usually relative to ds segment (in your case ds and cs registers are pointing to the same segment).

Tanana

Answer 4

In x86 16bit mode, address space is not flat; instead, addresses are composed of an offset and a "segment". The "segment" points to a 64K space, offset is within that space.

See http://en.wikipedia.org/wiki/Memory_segmentation

Camiecamila

Answer 5

From MASM Programmer's Guide 6.1 (Microsoft Macro Assembler)

The OFFSET Operator

An address constant is a special type of immediate operand that consists of an offset or segment value. The OFFSET operator returns the offset of a memory location, as shown here:

    mov     bx, OFFSET var  ; Load offset address

For information on differences between MASM 5.1 behavior and MASM 6.1 behavior related to OFFSET, see Appendix A.

Since data in different modules may belong to a single segment, the assembler cannot know for each module the true offsets within a segment. Thus, the offset for var, although an immediate value, is not determined until link time.

If you read carefully, the final value is determined after you "link" your object code to create a DLL/EXE. Prior to linking, all you have is an immediate value which represents the offset from the segment's base address.

Underage

Don't post images of text, if possible - instead, post the actual text. – Conjectural

Answer 6

Offset is basically the distance from the segment point(also called datum point). for example segment address is 0000 and the offset or logical address is 0100 then the physical address can be counted by adding the two pairs. Physical Address = 0000+0100=0100 Means that our required location is on the address of 0100. Similarly if segment address is 1DDD and offset is 0100 then : Physical address is : 1DDD+0100=1EDD

Means that our destination is 1EDD.

Polis