Computer Science 237
Computer Organization

Lecture 17: Synchronous Counters, Arrays in Assembly Language
Date: October 20, 2006

Announcements
- Pumpkin carving this afternoon, CS Lounge, 2:30
Counters
- Asynchronous: counted value eventually appears
- Synchronous: access to the new count appears all at once
- Think about: decade counters
Arrays in assembly
- allocating an array: make enough space for it on the stack: LINK
- address register indirect with displacement and indexing usage: d(\%an,\%dm.w)
  - %an is an address register containing the base address of the array (from LEA/PEA)
  - %dm is (word wide) register that provides the byte offset of the element from base address
  - d is selected to make index of first element 0 bytes away from base address
    - this element has various indices in various languages
    - Always 0 in C and Java - all arrays must be 0-based
    - but Pascal, for one, allows more general array declarations:
```
  a: array [1..20] of integer;
```
      or even
```
  a: array [-10..10] of integer;
```
      In the first case, this "correction factor" would be -2, to offset the indexed value by -2 (subtract 1 2-byte element). In the second case, we have a correction factor of 20 (add 10 2-byte elements).