This program is copyright and may not be used, copied or otherwise distributed without the permission of the author.
Using the Emulator
------------------
The program is started by double clicking on the !Cambridge icon. The icon should then appear on the icon bar. Two types of file can now be loaded :-
1. Source code files which are then assembled ready for use, or
2. Object code files saved from a previous session.
There may be a small delay while source code files are assembled.
Clicking select on the icon will now show the main window. On the left of the window is the next instruction to be executed or the message 'Click on Start', the current contents of the Accumulator and the X register and the start/stop button.
The next instruction display is in the follwing form:
00 - 3E FF - LDA #&FF
| | | |
| | | |- This is the mnemonic
| | |
| --- This is the contents
|
|- This is the address
On the right of the display is the current Input/Output status.
The inputs can be altered by clicking on them. (Pushed in means that they are on)
The Input/Output status is displayed in hex as well above the displays.
If you click with menu on the icon bar icon, a menu will appear from which the following options are available:
Info - leads to a dialogue box with information about the program
Save - leads to a standard save box to save the object code
Speed - allows you to set the speed in units of 1/10th of a second
(min 0.1s)
Reset - Sets A, X, and PC to 0.
Quit - Quits program
The source files
----------------
The format for instructions is as follows:
<mnemonic> [parameters]
The mnemonic is one of those described in Appendix A. The parameters are dependent on the mnemonic and some instructions, eg INX, do not require any.
Memory addresses are typed normally, eg LDA &34 will load the accumulator from address &34. Immediate constants should be preceded with a # eg LDA #4 will load the accumulator with the value 4.
Comments may be started with either the ';' character or the '\' character and last until the next line.
To make programs more readable, the MAKE command allows you to name numbers, eg
MAKE Output &FF
will allow you to use 'Output' instead of &FF anywhere in the program, eg
STA Output
It is important not to MAKE a variable with the same name as a label.
Labels can be created with .<name> and then referred to as <name> eg
.Loop
JMP Loop
To define a series of bytes as numbers use DEFB, eg
DEFB &65 &64 &63 .
(Note the full stop on the end)
Whenever a number is required, a calculation may be used, eg
lda #&45+2
Hex numbers are recognised by the & sign.
Decimal numbers are used by default.
Binary numbers can be entered by preceding them with %, eg %11010100
Variables made with the MAKE comand and labels may not be used in calculations, eg
MAKE Input &3E
MAKE Output &3E+16
STA Output ; This is fine
MAKE Input &3E
MAKE Output Input+16 ;This will cause an error since Input was
STA Output ;defined with MAKE
Points to remember:
- Everything is case insensitive.
- MAKE values and labels must never begin with the letter X, or they'll get confused
with the X register
That sums up the brief guide. I'll do a full guide on Impression sometime with details on how to write device drivers sometime...
Appendix A - Instructions
-------------------------
M(n) means memory location n.
M(X) means memory location held in X register.
The microprocessor has 256 bytes of memory.
There is an 8bit input port at address &EF and an 8bit output port at address &FF.
The processor has 3 8bit registers:
• Program counter
• Accumulator (A)
• Index register (X)
Instruction Function Mnemonic
----------- -------- --------
3E n A = n LDA #n
C6 n A = A + n ADD #n
D6 n A = A - n SUB #n
E6 n A = A AND n AND #n
EE n A = A EOR n EOR #n
3A n A = M(n) LDA n
32 n M(n) = A STA n
7E A = M(X) LDA X
77 M(X) = A STA X
86 A = A + M(X) ADD X
96 A = A - M(X) SUB X
A6 A = A AND M(X) AND X
AE A = A EOR M(X) EOR X
6F X = A TAX Transfer A to X
7D A = X TXA Transfer X to A
C9 X = X + 1 INX
C3 n PC = n JMP n
CA n PC = n if A = 0 JEQ n
C2 n PC = n if A<>0 JNE n
00 Do nothing NOP
76 Halt HLT
The instructions are 1 byte long when assembled except when a parameter is required in which case they are two bytes long.
The instruction set is exactly as specified by the Cambridge Exam board.
This copy of !Cambridge also includes three device drivers:
EgDrive - an example of how to write device drivers which replaces the normal input and
