301 Spacelizard APU :: Quicker, easier and cheaper to make your own chip!

301 : Spacelizard APU

Author: Steve W.

Description: Tiny 8-bit arithmetic processing unit with registers, flags, RAM, and bus interface

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Clock: 1000000 Hz

How it works

Spacelizard APU is a tiny 8-bit arithmetic processing unit.

It contains:

8-bit accumulator register A
8-bit register B
flags: zero Z, carry C, negative N, overflow V
8-byte internal RAM
arithmetic, logic, shift, and rotate operations

The control interface uses ui_in:

ui_in[7] = EXEC strobe
ui_in[6:4] = opcode
ui_in[3:0] = argument

The data bus uses uio_in and uio_out.

uo_out[7:0] always shows the current value of register A.

RAM uses only arg[2:0], so addresses are 0 to 7.

Instruction summary

Opcode	Function
`000`	load/move/clear registers
`001`	arithmetic: add/sub
`010`	logic: and/or/xor/not
`011`	shifts/rotates
`100`	write RAM
`101`	read RAM into `A`
`110`	read register/status to `uio_out`
`111`	read RAM directly to `uio_out`

How to test

Apply a command on ui_in, put data on uio_in if needed, then pulse EXEC high for one clock cycle.

Example: load A = 0x12

Set uio_in = 0x12
Set ui_in = 0b10000000
Clock once
Set ui_in = 0b00000000
uo_out should show 0x12

Example: add B into A

Load A
Load B
Pulse command opcode = 001, arg = 0
Result appears on uo_out

The included cocotb test can be run with:

make -C test

#	Input	Output	Bidirectional
0	ARG0	A0	DATA0
1	ARG1	A1	DATA1
2	ARG2	A2	DATA2
3	ARG3	A3	DATA3
4	OP0	A4	DATA4
5	OP1	A5	DATA5
6	OP2	A6	DATA6
7	EXEC	A7	DATA7

Input

Output

Bidirectional

ARG0

DATA0

ARG1

DATA1

ARG2

DATA2

ARG3

DATA3

OP0

DATA4

OP1

DATA5

OP2

DATA6

EXEC

DATA7

Chip location

301 Spacelizard APU