wizzygeek / tisc

A Tiny cute CPU

(This version is not complete)

TeenyISC

Teeny.I.S.C or Teeny Instruction set computer is my own IA designed for learning

The instruction set is as compact as possible It consists of

• jmp jz jc
• hlt nop
• add sub nand copy
• movx
• rloadrl rstorerl loadrl storerl
• rloadi loadi

This implementation doesn't concern itself with the implementation of the IA as it would be physically for now

This project provides a simulation, assembler, disassembler. All 3 are currently under development but are ready for use. However before using you will have to put togather your simulation by assembling the components in python (CLI and utilities in works).

Architecture

Tisc has a very basic architecture which is not designed with real world usage in mind.

Registers

TISC is a register machine with 6 developer accessible register which are further divided into 4 standard and 2 result registers.

	0	1
00	x	y
01	z	w
10	rl	rh
11	ipl	iph

x's register code is 000, y's is 001 and so on, the last two registers can not be accessed.

x, y, z, w are standard registers, note how these registers can be addressed using 2 bits only

Pins

The TISC CPU defines a 16 bit address bus, a seperate 8 bit pin and an extra r pin which maybe used as the 17th address bit or as it was intended to be used, that is as a selector between ROM and RAM. The r pin is directly controlled by the programmer by specifying the r pin state in the IO instructions At maximum, the first tisc version (this one) can only address 128 KiB memory

ALU

The Arithmetic and Logic unit is a small part with only 4 capabilities, copying, adding, substractinga and nand-(ing). ALU opcodes are

{
    "add": 0,
    "sub": 1,
    "nand": 2,
    "copy": 3,
}

Instructions

• jmp

  The jmp instruction has the following format 00000010 [offset LSB] [offset MSB] it is a relative jump with the address being stored in little endian

• jz

  The jz instruction is written as 00010010 [offset LSB] [offset MSB] It jumps only if the last executed ALU instruction resulted in a zero.

• jc

  Written as 00100010 [offset LSB] [offset MSB] It jumps only if the last executed ALU instruction resulted in a carry/borrow

• hlt

  HALT stops the machine, written as 00001111

• nop

  No operation, simply increment instruction pointer.

• add

  add two register contents and store the result in first register. 0100xxyy xx is the 2 bit register code for the first register while yy is the register for the second register

• sub

  Subtract two register contents and store result in first register. Written as 0101xxyy

• nand

  nand two register's content and store in first register. written as 0110xxyy

• copy

  copy 2nd register's yy value into first register xx. Written as 01110000

• movx

  Swap two register's contents. written as 10xxxyyy xxx, yyy are 2 3bit register codes for 2 registers invluding the result registers

• (r)loadrl

  rloadrl or loadrl loads the memory pointed to by the rl-rh register pair into the specified register. Written as 110xxx0r xxx is the 3 bit register code, r is the r pin output.

• (r)storerl

  rstorerl or storerl stores the value in the specified register to the location described by rl-rh register pair. Written as 111xxx0r xxx is the 3 bit register code, r is the r pin output.

• (r)loadi

  rloadi or loadi Loads the value from the instruction pointer, typically from program memory into the specified register. Written as 110xxx1r xxx is the 3 bit register code, r is the r pin output.

TASM

TASM, TeenyASM is a lightweight extremely primitive assembly for TISC. Syntax will be documented here soon

DISTASM

DISTASM is the disassebler for compiler TASM memory files, currently it gives only a rudimentary output

tasmc.bitfield

This is a submodule specific to Tasm, it simplifies packing of bits into python integers

License

This software is released free of charge without any warranties or guaranties. The author reserves all rights. (For now)