An 8 bit OpenSource full computer

This is Libre 8, an 8 bit OpenSource computer.

Inspired by the SAP-1 computer (Simplest As Possible). Contains 4 8-bit registers and it is able to address 24-bit memory addresses, having a total ammout of 16MB of RAM. The designs were made with Logisim, the microcode and the compiler are writen in Java. The simulation is able to reach up to 4.1 Khz.

Currently must be programmed using Assembly like operations.

;; This is a comment
;; First thing is to write .data, and variables and function offset declaration below.
.data

;; Here we can define variables or function offsets.
;; A variable declaration is as following:
;; <name> <address> <value>
one 0000ffh 01

;; A function declaration is the same without value
addOne 0001ffh

;; To start coding you have to write .code (beginnings of the program)
.code
;; Similar to .code, you can define a tag using "."
;; We are now defining .Main tag. (saving the current offset of the execution in the name of the tag)
.Main
;; We will load value of 'one' var (01) to A and ADD one to its value.
LDA one
;; We call addOne function
B addOne
;; Ant print it on screen
OUTA

;; We define addOne function
.addOne
ADD one
STA result
BX result

You can also define running offset tags to help you to jump to (only backwards).

;; Same program as above with JNC to tag
.data
one 0000ffh 01
addOne 0001ffh
.code
.Main
LDA one
;; We define a running offset before the function call
.tag
B addOne
OUTA
;; We jump to tag to add on over and over again until carry flag is set.
JNC tag

.addOne
ADD one
STA result
BX result

To compile any code, you have to save it in a file and pass it as an argument to Compiler.java. You can also simply name it "main.as" and place it on root folder of the project. If you run
Compiler.java on eclipse it will find main.as file.

To run the simulation, open Libre8.circ with Logisim. The file resides inside logisim folder. Once the file is loaded, double click on labelled RAM component, (see picture of circuit) and load the program you previously compiled on the component

Then you can Enable simulation and run clock.

There are a total of 256 possible operation codes. Even you can create an operation code able to fetch complex instructions, formed by several opcodes concatenated.

So there is the other java file, Microcode.java. Within it are defined the instruction opcode in this format (0xab00). The opcode is just ab, but you need to write the trailing zeroes to make the program work.

Once you have your opcode, write the following inside the main method (anywhere just below any previpus create() invocations would do) the following sentences:

setOffset(OPCODE_CONST, icuadrant);
write(Signals.CO + Signals.MII);
write(Signals.RO + Signals.II + Signals.CE);
... your custom signaling goes here ...
write(Signals.clpcr);  

· CARRY is up when an overflow has occured while substracting or adding.
· ZERO is up when the ALU value is zero while substracting or adding.