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1. Introduction

The Completely Useless and Pointless Computer / 8 bit.

CUPC/8 is a simple 8-bit microcomputer designed and built by myself (David Kaplan) for fun over the years 2014-2016. The design is pretty dirty and 'evolved' as I added features necessary to achive the goal of a fully working, custom-designed 8-bit machine that could connect to the Internet. The full hardware and software stacks have been designed and coded from the ground up (with the exception of certain 3rd-party open source pieces of software whch I ported over to the CUPC/8 ISA). This document serves as a manual detailing all aspects of the computer for anyone that may, however unlikely, be interested in my work in the future.

The entire project is open-sourced under a Creative Commons Attribution-NonCommercial 4.0 International license (with the exception of 3rd-party software where other licenses may apply).

2. System Overview

2.1. Central Processing Unit SoC

The CUPC/8 microcomputer is made up of a Central Processing Unit (a simple SoC really) implemented in VHDL and running on an FPGA. The CPU has an 8-bit data bus and a 16-bit address bus and runs a custom Instruction Set Architecture (ISA). Coupled with the CPU is a Memory Management Unit (MMU) which is responsible for memory mapping external interfaces. The last component of the CPU SoC is some Serial Peripheral Interface (SPI) logic which is used to provide an interface with external peripherals. Other than a memory-mapped 8-bit output only register hardwired to LEDs on the PCB, peripherals can only connect to the microprocessor over the SPI bus.

2.2. Memory

As the CPU sports a 16-bit address bus, the maximum amount of RAM that can be addressed (without bank switching) is 64KB. This is provided by a external SRAM chip. This RAM chip is loaded on poweron from an external ROM chip which provides the initial code to be executed by the CPU. This is necessary due to the fact that there is no code within the FPGA SoC itself and the CPU can therefore not know how to load any code directly from ROM over the SPI bus. The current memory board design has a microcontroller on board which is responsible for copying ROM into RAM on poweron. Furthermore this microcontroller can interface to another external computer and allows for RAM and ROM to be written over UART.

2.3. Display

Display is piped over the SPI bus. Theoretically any display that works over SPI can be used (provided that a driver is writted for it). In reality, CUPC/8 neither has sufficient bandwidth over its SPI bus nor enough memory to drive large displays with many colours. The display chosen reflects these limitations.

2.4. Input


2.5. Software Stack

The software stack consists of a monolithic kernel which provides drivers for the SPI peripherals and filesystem, handles interrupts, etc. It also provides a limited set of fixed-vector 'libc-like' functions and graphic routines that can be used by applications. There is no separation between kernel- and user-space (it's more of a convention) and any application has full access to the entire memory space at any time. Ideally applications would take care not to overwrite kernel regions but there is nothing stopping them from doing so.

In addition to drivers and system functions, the kernel provides a 'terminal' interface which accepts a number of commands. A limited subset of BASIC is provided which allows BASIC programs to be executed directly from the terminal (in a non-interactive fashion).

There is no compiler available for the CUPC/8 ISA. Development tools are cross-platform and consist of an assembler and a simulator which provides 1-to-1 simulation of the full computer (including display and input). The simulator can be executed natively or compiled to JavaScript (using emscripten) and run through a web browser.

3. Central Processing Unit

3.1. Registers

The CUPC/8 has two 8-bit General Purpose Reigsters (GPRs), r0 and r1. These are available for use by arithmetic operations and LOAD/STORE operations.

A status register exists which provides the internal logic with flags that are set due to operations. The only flag currenty is the Z (zero) flag which is set during comparison operations (EQ/GT/LT) and allows the CPU to perform decisions based on the Z flag value (for example, BZF - which branches if the flag is not set).

Two internal 16-bit address registers are provided for program flow; the pc (program counter) and sp (stack pointer) registers. The pc register holds the address of the next instruction awaiting execution. The sp register points to the address of the next free chunk of the stack memory region. Neither of these two registers can be acted upon directly and are modified through program flow and stack operations only.

There are two special 8-bit registers, pcl and pch (the 'low' and 'high' program counter registers). These registers can be acted upon through the use of stack operations (i.e. PUSH/POP) only. The pch register is special in that upon receiving a value though a stack POP operation, the 8-bit values of the pcl and pch registers are copied into the pc register such that pc := pch||pcl and program flow jumps immediately to the new pc address. This is used to return from functions (as there is currently no dedicated RET instruction).

3.2. Memory layout

Region Start address End Address
Soft Reset Vector $0000 $0001
Reserved $0002 $000f
Interrupt Vector Table $0010 $00ff
Stack $0100 $0fff
Program $1000 $efff
I/O $f000 $ffff
Soft Reset Vector (SRV) Region

On boot, the CPU will start executing from the Hard Reset Vector, $f000 (i.e. pc := $1000) and sp will be reset to the bottom of the stack (sp := $0100). A configurable SRV can be set at $0000. Even if unused this vector should be set to $1000 as soon as possible after boot.

Interrupt Vector Table (IVT) Region

The kernel is responsible for setting up the IVT to handle interrupts that may occur from external sources (such as peripherals). [TODO]

Stack Region

The stack is an 8-bit stack which can be accessed by PUSH/POP operations. The architecture does not support addressing off the stack pointer (there is no way to access something like [sp + $10] for example) or block allocation of stack regions (there are no base/frame pointers). Warning: Overflowing the stack is completely possible and the CPU will not care - so don't do this!

Program Region

This is where the code goes. The CPU will start executing from $1000 and the CUPC/8 kernel places a jump to the kernel initialization function vector at this address.

Input/Output Region

The MMU is responsible for mapping peripherals to the memory space. Currently this region is used for the General Purpose Output (GPO) pins (8-bit mapped to $f000) and the SPI bus (mapped at $f1XX).

3.3. Input/Output

General Purpose Output

GPOs are mapped to $f000, a simple 8-bit STORE operation to this address will set the pin states accordinly.

Serial Peripheral Interface Bus

The CPU can act as an SPI master for up to 4 SPI slaves. Each SPI controller is mapped to $f1X0-$f1Xf, where X denotes the device number (0-3).

The low nybble of the address denotes the SPI operation to be performed via interactions with the following registers:

Address Operation Descrption
$f1X0 STORE Write byte to TX buffer
$f1X1 LOAD Read byte from RX buffer
$f1X2 STORE Perform transaction
$f1X3 LOAD Get SPI status
$f1Xf STORE Configure SPI

SPI master configuration is done via a STORE to the configuration register, $f1Xf. The register configuration bits are as follows:

00000 0 0 0
|   | | | |
 ---  | |  ----> continous
  |   |  ------> cpol
  |    --------> spha
   ------------> clk_div

The continous, cpol and cpha bits are self-explanatory to anyone familiar with SPI. clk_div allows the user to select the SPI bus speed as a factor of the system clock speed. The 5-bit value allows for a division of up to 31 such that a clk_div value of 1 will run the SPI bus at the same speed as the system clock.

After configuration of the SPI device, a typical transaction might look as follows:

	mov r0, #255
	st $f100, r0	; write 0xff to TX buffer
	mov r0, #1
	st $f102, r0	; perform transaction by writing 1 to $f10X2
	eq r0, #0
	bzf .transaction_done
	ld r0, $f103	; see if SPI is busy
	b .loop
	ld r0, $f101	; read RX buffer

3.4. Instruction Set Architecture

The CUPC/8 instruction set contains of 38 8-bit instructions. Each instruction consists of an 5-bit/6-bit operation code (opcode) followed by register bits and zero, one or two bytes as defined by the instruction format.

Register Format

Instructions of the register format act on registers only and are 8-bit in length. The 5 most significant bits 7..3 denote the operation, bit 2 should always be cleared, bits 1 and 0 are registers Rb and Ra respectively (bit cleared = r0, bit set = r1).

The CPU performs only a single fetch operation for instructions of this formations are the fastest to execute.

[   ins   |  0  |  Rb  |  Ra  ]
   {7..3}   {2}    {1}   {0}
Immediate Format

Instructions of the immediate format are used when operations are performed on immediate 8-bit values. The most significant 5 bits denote the operation. Bit 10 must always be set. Bit 9 is ignored. Bit 8 denotes register Ra (bit cleared = r0, bit set = r1). The final 8 bit value is the immediate value.

The CPU performs two fetch operation for instructions of this format.

 --------------------------------    -------
[    ins    |   1   |  ?  |  Ra  ]  [  IMM  ]
 --------------------------------    -------
   {15..11}   {10}    {9}    {8}      {7..0}
Memory Format

Instructions of the memory format are used to store and load values via the MMU. The 5 most significant bits denote the operation. The next bit is ignored. Bits 17 and 16 are the Rb and Ra registers respectively. The final 16 bits denote the address of the memory operation.

The CPU performs three fetch operation for instructions of this format. These, together with the flow format instructions, are the slowest instructions to execute.

 -------------------------------------    ----------
[    ins    |   ?   |   Rb   |   Ra   ]  [   ADDR   ]
 -------------------------------------    ----------
   {23..19}   {18}     {17}     {16}       {15..0}
Flow Format

Instructions of the flow format are used to modify program execution flow (such as by modification of the program counter). The most significant 5 bits denote the operation. The next 3 bits are ignored. The final 16 bit value holds the target address for the program flow change operation.

The CPU performs three fetch operation for instructions of this format. These, together with the memory format instructions, are the slowest to execute.

 -----------------------    ----------
[    ins    |     ?     ]  [   ADDR   ]
 -----------------------    ----------
   {23..19}   {18..16}        {15..0}
Mneumonic Opcode Format Syntax Operation
MOVr 88 R MOV Ra, Rb Ra <= Rb
MOVi 8c I MOV Ra, #imm Ra <= imm
PUSHr 90 R PUSH Rb [SP] <= Rb; SP <= SP + 1
PUSHi 94 I PUSH #imm [SP] <= imm; SP <= SP + 1
POP 98 R POP Ra Ra <= [SP]; SP <= SP - 1 (; PC <= PCH||PCL on Ra = PCH)
LD a0 M LD Ra, $addr Ra <= [addr]
LDind a4 M LD Ra, $addr+Rb Ra <= [addr + Rb]
ST a8 M ST $addr, Rb [addr] <= Rb
STind ac M ST $addr+Ra, Rb [addr + Ra] <= Rb
LDD 70 M LDD Ra, $addr Ra <= [[addr]]
LDDind 74 M LDD Ra, $addr+Rb Ra <= [[addr + Rb]]
STD 78 M STD $addr, Rb [[addr]] <= Rb
STDind 7c M STD $addr + Ra, Rb [[addr + Ra]] <= Rb
B b0 F B $addr PC <= addr
BZF b8 F BZF $addr if ZF: PC <= addr
EQr 00 R EQ Ra, Rb ZF <= Ra == Rb
EQi 04 I EQ Ra, #imm ZF <= Ra == imm
GTr 08 R GT Ra, Rb ZF <= Ra > Rb
GTi 0c I GT Ra, #imm ZF <= Ra > imm
LTr 10 R LT Ra, Rb ZF <= Ra < Rb
LTi 14 I LR Ra, #imm ZF <= Ra < imm
ANDr 18 R AND Ra, Rb Ra <= Ra & Rb
ANDi 1c I AND Ra, #imm Ra <= Ra & imm
ORr 20 R OR Ra, Rb Ra <= Ra | Rb
ORi 24 I OR Ra, #imm Ra <= Ra | imm
XORr 30 R XOR Ra, Rb Ra <= Ra ^ Rb
XORi 34 I XOR Ra, #imm Ra <= Ra ^ imm
NORr 38 R NOR Ra, Rb Ra <= ~(Ra | Rb)
NORi 3c I NOR Ra, #imm Ra <= ~(Ra | imm)
ADDr 40 R ADD Ra, Rb Ra <= Ra + Rb
ADDi 44 I ADD Ra, #imm Ra <= Ra + imm
SUBr 48 R SUB Ra, Rb Ra <= Ra - Rb
SUBi 6c I SUB Ra, #imm Ra <= Ra - imm
SHLr 60 R SHL Ra, Rb Ra <= Ra << Rb
SHLi 64 I SHL Ra, #imm Ra <= Ra << imm
SHRr 68 R SHR Ra, Rb Ra <= Ra >> Rb
SHRi 6c I SHR Ra, #imm Ra <= Ra >> imm
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