Computer Architecture Lab/WS2007/Project -1 Lab4

Lucky Processor

Thank you for choosing the "Lucky, das Ding aus dem See" Processor!

This especially for the Cyclone II - EP1C12Q240C8N FPGA designed Processor will revolutionize the gambling-world.

Features

32-bit RISC architecture
32-bit fixed instruction length
24 instructions
32 32-bit registers
Groundbreaking special function (Hardware implemented Random)
4-stage in-order pipeline
Simple UART communication
Program and data-memory are separated (Harvard)

Instruction Set Architecture

We chose the RISC-common Register-Register (or load/store) Instruction Set Architecure with a fixed length of 32-bit per instruction.

Registers

The processor has 32 32-bit registers, which are mapped into the internal FPGA memory due to performance and size reasons.

Register	Function
$0x00	Is always 0
$0x01 ... $0x08	Local Variables (Pushed to Stack by Functions)
$0x09 ... $0x19	Global/Temporary Variables, must watch consistency for yourself,
$0x09	Function Return by convention
0x09,0x0A..	Calling Parameters by convention
$0x1A	Reserved for later use in Pseudo Functions
$0x1B	Temporary Register for Pseudo Calls
$0x1C	Temporary Register for Pseudo Calls
$0x1D	Pseudo Stack Pointer
$0x1E	Reserved for Randomizer
$0x1F	Contains current PC value

Flags

Our architecture can set 4 flags:

Negative
Overflow
Zero
Random

The first three flags are set immediately in the ALU after an instruction was executed, the Random flag can change its status every cycle.

Instructions

There are:

13 Arithmetical/Logical Instructions
3 Load/Store Instructions
8 Branch and Jump Instructions

The Instruction Set is described in detail here.

Addressing Modes

The Lucky Processor provides two types of addressing:

Register
Immediate

Special Instructions

Because of the integrated Randomizer that provides a new Random value and RandomFlag every cycle, we have the special function Branch on Random (BOR) that allows the influence of the program behaviour in a (controlled) random way! i.e. for Casino games this feature is a good tool. If you need a random 32-bit value you just haev to read the content of Register $1E that contains a new random value every cycle.

Randomizer

This is the heart of the provided special function of our Lucky Processor. It has an integrated Randomnumbergenerator that uses the TLV3502 rail-to-rail comparator on the dspio-boeard, a modified SigmaDelta AD Converter (from Opencores) and a LFSR.

A 32-bit random value is stored in the register $1E and updated every cycle. Additionally the Random Flag is updated every cycle and allows, in addition with the special function BOR (Branch On Random-bit set), a new, unpredictable program behaviour everytime you run the same program.

To read the Random-Register and view the output:

ADD("1","0","1E");
DEBUG_SEND("1",&pc);

The Branch on Random function:

BOR(LABEL);

A testrun including BOR showed, that the distribution of 1s and 0s in the Random Flag is good: 26116 vs. 26078

sourcefile for the test

Memory Access

The internal FPGA memory is used for I/O access and the RAM.

Mapping for I/O access:

Address	I/O Function
0x001	Button0
0x002	Button1
0x004	UART: rxd has new value
0x005	UART: txd ready to send
0x006	UART: get RXD byte
0x009	WD
0x00A	LED0
0x00B	LED1
0x00C	UART: txd send now
0x00E	UART: set txd byte

The address space for our RAM is from 0x00F to 0xFFF

UART Communication

The Lucky Processor allows a simple way to communicate with its internal UART. The easiest way to send something is to use the function:

UART_SEND_STRING("Hello World! ",&pc);

This codeline will result in writing "Hello World! " to your HyperTerminal interface.

For more details for the common UART:

Send

If UART txd ready to send is set, it indicates that a new byte, which is in UART set txd byte, can be sent. UART txd send now triggers sending. When it is set (1) you can send, if it is cleared (0) you can't send. If you write a 1 to UART txd sed now while it still send (is set to 1) it will have no effect beacuse it has to be cleared before you can send again.

Receive

If UART rxd has new value is set, it indicates that UART get rxd byte has a new value to read. If UART rxd has new value is cleared UART get rxd byte has bean read at least once.

If you read from a memory location that is dedicated to write e.g. read from UART set txd byte, it will return 0 not the value it really contains. If you write to a location that is dedicated to read it will have no effect.

Pipeline

The 4 Stages in the Lucky Processor are the following:

Instruction Fetch

Here the Instructions are fetched from the ROM into the instruction registers and the Program Counter is incremented.

Instruction Decode

This stage decodes the Instructions into ops, operands and immediate values.

Register Fetch

Fetch special- and General-Purpose-Register values.

Execute/Writeback

Here the Instructions are executed and the results are written back to the Registers.

Hazard Strategies

Data hazards: In our architecure RAW (Read after Write) data hazards are avoided by forwarding, and because we have an in-order pipeline WAW and WAR cannot occur.
Control hazards: Register contol hazards cannot occur because we use a 3-way memory for our registers.
Conditional hazards: We chose the "predict Branch not taken" approach to deal with these types of hazards. In case of a misspredict the instructions of the next 4 cycles are dumped.

Assembler

Source

The source of our assembler can be found here

Syntax

The assembler syntax including instruction samples can be found here

The following instructions are implemented using the processors instructions, they are not part of the processors isa.

Pseudo Stack

The register 0x1D is used as a stack pointer. It is initialized to the last memory adress (0x1FF).

Pseudo Call

A Call is translated into 3 instructions, it branches into a special call handler which takes 23 instructions to push the local variables (reg 0x01-reg 0x08) and the return adress on the pseudo stack (the stackpointer is decremented). The new stack content after a call is:

[lower adress] [return adress] [r0x08] [r0x07] ... [r0x01] [oldStackContent] [0x1FF]

The stack pointer points to the [return adress].

Pseudo Return

A Return is translated into a single instruction, it branches into a special return handler which takes 20 instructions to pop the local variables (reg 0x01-reg 0x08) and the return adress from the pseudo stack (the stackpointer is incremented). The new stack content after a call is: [br] [lower adress] [oldStackContent] [0x1FF] After the return the stack pointer points to the [return adress] of the oldStackContent.

Example Programs

Example program files can be found here

Next Steps

Because of the easy instruction set the next version of the processor would have no decoder stage.