Computer Architecture Example

CSE 410 Fall 98

Memory

There is a memory of 1000 locations, numbered from 0 to 999. Each location is capable of holding a number. Since instructions are numbers, each location is also capable of holding an instruction.

Processor Registers

The processor contains the following special locations, called registers.

The accumulator, where all arithmetic operations take place.
The program counter, (also called the PC), which contains the location number of the next instruction to be executed.
The instructor register, which contains the instruction being currently executed.

Instructions

Each instruction consists of a 1-digit opcode followed by a three-digit location number, also called the address. There are 9 opcodes, listed below.

Opcode	Mnemonic	Description	Notes
0	halt	Halt the computer	(address is ignored)
1	load	Load from location	accum<-location
2	store	Store to location	location<-accum
3	add	Add	accum<-accum+location
4	sub	Substract	accum<-accum-location
5	mul	Multiply	accum<-accum*location
6	div	Divide	accum<-accum/location
7	br	Branch to address	pc<-address
8	brpos	Branch if accum>0	if (accum>0) pc<-address

Example: compute y=x*x+4*x-5

10:	1100		accum <- x
11:	5100		accum <- x*x
12:	2019		temporary <- x*x
13:	1100		accum <- x
14:	5020		accum <- 4*x
15:	4021		accum <- 4*x-5
16:	3019		accum <- x*x+4*x-5
17:	2101		y <- accum
18:	0000		halt
19:	0000		(temporary location)
20:	0004		(constant 4)
21:	0005		(constant 5)