ocra2
=====

This is a simple emulator for the (fake) microprocessor described by OCR in their A2 Electronics exam (until 2009). It reads a binary file with instructions and runs the program on the host machine in an emulated environment.

During the emulation the instruction being executed will be printed in a human readable format (as described by the instruction set). 

@author Tom Hennigan <tomhennigan@gmail.com>
@year 2008

ocra2 Details
=============
Information from the insert provided by OCR (http://www.ocr.org.uk/Data/publications/past_papers_2005_june/L_A_Level_Electronics_2530_Jun_2005_Insert.pdf)

The microprocessor has the following registers 
Program counter (8 bit) PC 
Accumulator (8 bit) A 
Index Register (8 bit) X 
M(n) represents the contents of memory whose address is the value of the byte n. 
M(X) represents the contents of memory whose address is stored in the index register. 
M(FF) is an eight bit output port; M(EF) is an eight bit input port. 

INSTRUCTION FUNCTION 

3E	n	A <- n
C6	n	A <- A + n
D6	n	A <- A - n
E6	n	A <- A & n
EE	n	A <- A eor n

3A	n	A <- M(n)
32	n	M(n) <- A

7E		A <- M(X)
77		M(X) <- A

86		A <- A + M(X)
96		A <- A - M(X)
A6		A <- A & M(X)
AE		A <- A eor M(X)

6F		X <- A
7D		A <- X
C9		X <- X + 1

C3	n	PC <- n
CA	n	PC <- n if A == 0
C2	n	PC <- n if A != 0

00		No Operation
76		Halt

The first instruction fetched by the microprocessor after a reset will be the one held in the memory 
location whose address is 00.