8-BIT-TTL-CPU

Step by step documentation of building a 8 bit TTL CPU from 74xx logic chips.

Story

Usually for beginners or non-technical guys, its hard to understand how a CPU operates and executes the program.

There should be a way to understand on how a basic how individual instructions are executed, which gives a fair bit of idea to us. So I decided to make a CPU on breadboards, using 74 series TTL logic chips which are easy to find.

I decided to make a 8 bit CPU that just runs on some Hz frequency (Not GHZ as we want to see what it does), which will help understanding fundamentals of CPU Operations.

The architecture

The SAP-1 computer is a bus-organized computer and makes use of Von-Neumann architecture. It makes use of an 8-bit central bus and has ten main components. A pictorial representation of its architecture is shown below.

The instruction set

This CPU supports 4 bit instructions, which are the following -

INS	OPCODE	OPERATION
NOP	0x0	No operation
LDA	0x1	Copy content from RAM at the address to A reg.
ADD	0x2	Add content of RAM at the address to A reg and store in A reg
SUB	0x3	Subtract content of RAM at the address to A reg and store in A reg
STA	0x4	Store content of A reg in RAM at the address
LDI	0x5	Load A reg immediatly with the 4 bit value
JMP	0x6	Jump to the address
JC	0x7	Jump to the address if carry flag set
JZ	0x8	Jump to the address if zero flag set
--	0x9	--
--	0xa	--
--	0xb	--
--	0xc	--
--	0xd	--
OUT	0xe	Copy content from A reg to Output reg
HLT	0xf	Halt the CPU

CPU Modules

Generating machine code

Writing the machine code from assembly code manually is a tedious task. To make life easy, use the assembler.py script to do the same job quickly.

Usage

python assembler.py --asmfile="<ASSEMBLY-FILE>"

Sample programs can be found here -> asm-files

Programming the microcode EEPROMs

You can find a seperate repository with code, schematics & instructions for programming the microcode EEPROMs here

List of alterations

1. For manual clock pulse increase the value of timing capacitor. If the push button switch is switching more than once in the pulse duration, it will trigger the clock pulse more than once, for a single step. So increase the value of timing capacitor. I used 0.44 uF capacitor.

2. I connected a 1k resistor between select input of the 74157s on the data selectors, in the RAM module.

3. Originally I used 74273 (Octal D-Type Flip Flop) along with a AND gate as his output register. This has some issues & it will generally latch in any value that's coming on the bus even if OI signal is not active. To solve this just simply use two 74173s to make a 8 bit register (Same as A & B register).