What does the ORG directive do in assembly code?

Question

For reference, the code is for the motorola 68008.

Say I have code such as the following:

org 200
sequenceO: ds.b 5
sequenceN: ds.b 5

move.w #sequenceO, A0
move.w #sequenceN, A1

1. Am I correct in thinking that A0 will hold the value 200 and A1 the value 205?

2. One of the exam questions in a past paper was: "What are the physical addresses of sequence0 and sequenceN?", would the answer be "200 and 205", or would it be "200-204 and 205-209"?

3. I've seen a few pieces of code with multiple org directives, eg;

   org 100

   array1: ds.b 4

   org 300

Am I right in thinking that the last org directive is followed, eg in this case, array1 points to 300?

Answer 1

1. Yes, that sounds right. The address of sequenceN is 5 bytes beyond sequence0.
2. "That depends", I guess ... Since it's "addresses" in plural, I guess they wanted the entire range, in which case your latter answer is correct.
3. No, I would expect multiple orgs to just apply to the code following them, so in that case the array1 would be at $100. Since no code or data generation happens after the latter org, it's basically ignored by the assembler.

Answer 2

I assume that "ORG" means "origin" - the first address to be assigned to the code or data segment being emitted.

Answer 3

1. Yes, 200 and 205

2. Sequence0 starts at 200 and extends for 5 bytes to 204

3. No, array1 starts at 100, anything after the org 300 would start at 300

Answer 4

You're using: MOVE.W #sequenceO, A0

So, you're loading only the lower word (16 bits) of the address into A0. That'll only work in very low memory ( A0 under $00010000)

In general using a MOVE.W on an address register gets tricky.

Try: LEA #sequence0, A0 (loads a 32-bit address into A0)

Most assemblers will also do:

MOVEA.L #sequence0, A0

Thanks, Dave Small