Subversion Repositories zipcpu

///////////////////////////////////////////////////////////////////////////////

//

// Filename:    cpudefs.v

//

// Project:     Zip CPU -- a small, lightweight, RISC CPU soft core

//

// Purpose:     Some architectures have some needs, others have other needs.

//              Some of my projects need a Zip CPU with pipelining, others

//      can't handle the timing required to get the answer from the ALU

//      back into the input for the ALU.  As each different projects has

//      different needs, I can either 1) reconfigure my entire baseline prior

//      to building each project, or 2) host a configuration file which contains

//      the information regarding each baseline.  This file is that

//      configuration file.  It controls how the CPU (not the system,

//      peripherals, or other) is defined and implemented.  Several options

//      are available within here, making the Zip CPU pipelined or not,

//      able to handle a faster clock with more stalls or a slower clock with

//      no stalls, etc.

//

//      This file encapsulates those control options.

//

//      The number of LUTs the Zip CPU uses varies dramatically with the

//      options defined in this file.

//

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

///////////////////////////////////////////////////////////////////////////////

//

// Copyright (C) 2015, Gisselquist Technology, LLC

//

// This program is free software (firmware): you can redistribute it and/or

// modify it under the terms of  the GNU General Public License as published

// by the Free Software Foundation, either version 3 of the License, or (at

// your option) any later version.

//

// This program is distributed in the hope that it will be useful, but WITHOUT

// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or

// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

// for more details.

//

// License:     GPL, v3, as defined and found on www.gnu.org,

//              http://www.gnu.org/licenses/gpl.html

//

//

///////////////////////////////////////////////////////////////////////////////

`ifndef CPUDEFS_H

`define CPUDEFS_H

//

//

// The first couple options control the Zip CPU instruction set, and how

// it handles various instructions within the set:

//

//

// OPT_ILLEGAL_INSTRUCTION is part of a new section of code that is supposed

// to recognize illegal instructions and interrupt the CPU whenever one such

// instruction is encountered.  The goal is to create a soft floating point

// unit via this approach, that can then be replaced with a true floating point

// unit.  As I'm not there yet, it just catches illegal instructions and

// interrupts the CPU on any such instruction--when defined.  Otherwise,

// illegal instructions are quietly ignored and their behaviour is ...

// undefined. (Many get treated like NOOPs ...)

//

// I recommend setting this flag, although it can be taken out if area is

// critical ...

//

`define OPT_ILLEGAL_INSTRUCTION

//

//

//

// OPT_MULTIPLY controls whether or not the multiply is built and included

// in the ALU by default.  Set this option and a parameter will be set that

// includes the multiply.  (This parameter may still be overridden, as with

// any parameter ...)  If the multiply is not included and

// OPT_ILLEGAL_INSTRUCTION is set, then the multiply will create an illegal

// instruction that will then trip the illegal instruction trap.

//

//

`define OPT_MULTIPLY    1

//

//

//

// OPT_DIVIDE controls whether or not the divide instruction is built and

// included into the ZipCPU by default.  Set this option and a parameter will

// be set that causes the divide unit to be included.  (This parameter may

// still be overridden, as with any parameter ...)  If the divide is not

// included and OPT_ILLEGAL_INSTRUCTION is set, then the multiply will create

// an illegal instruction exception that will send the CPU into supervisor

// mode.

//

//

`define OPT_DIVIDE

//

//

//

// OPT_IMPLEMENT_FPU will (one day) control whether or not the floating point

// unit (once I have one) is built and included into the ZipCPU by default. 

// At that time, if this option is set then a parameter will be set that

// causes the floating point unit to be included.  (This parameter may

// still be overridden, as with any parameter ...)  If the floating point unit

// is not included and OPT_ILLEGAL_INSTRUCTION is set, then as with the

// multiply and divide any floating point instruction will result in an illegal

// instruction exception that will send the CPU into supervisor mode.

//

//

// `define      OPT_IMPLEMENT_FPU

//

//

//

// OPT_NEW_INSTRUCTION_SET controls whether or not the new instruction set

// is in use.  The new instruction set contains space for floating point

// operations, signed and unsigned divide instructions, as well as bit reversal

// and ... at least two other operations yet to be defined.  The decoder alone

// uses about 70 fewer LUTs, although in practice this works out to 12 fewer

// when all works out in the wash.  Further, floating point and divide

// instructions will cause an illegal instruction exception if they are not

// implemented--so software capability can be built to use these instructions

// immediately, even if the hardware is not yet ready.

//

// This option is likely to go away in the future, obsoleting the previous

// instruction set, so I recommend setting this option and switching to the

// new instruction set as soon as possible.

//

`define OPT_NEW_INSTRUCTION_SET

//

//

//

//

//

//

// OPT_SINGLE_FETCH controls whether or not the prefetch has a cache, and 

// whether or not it can issue one instruction per clock.  When set, the

// prefetch has no cache, and only one instruction is fetched at a time.

// This effectively sets the CPU so that only one instruction is ever 

// in the pipeline at once, and hence you may think of this as a "kill 

// pipeline" option.  However, since the pipelined fetch component uses so

// much area on the FPGA, this is an important option to use in trimming down

// used area if necessary.  Hence, it needs to be maintained for that purpose.

// Be aware, though, it will drop your performance by a factor between 2x and

// 3x.

//

// We can either pipeline our fetches, or issue one fetch at a time.  Pipelined

// fetches are more complicated and therefore use more FPGA resources, while

// single fetches will cause the CPU to stall for about 5 stalls each 

// instruction cycle, effectively reducing the instruction count per clock to

// about 0.2.  However, the area cost may be worth it.  Consider:

//

//      Slice LUTs              ZipSystem       ZipCPU

//      Single Fetching         2521            1734

//      Pipelined fetching      2796            2046

//      (These numbers may be dated, but should still be representative ...)

//

// I recommend only defining this if you "need" to, if area is tight and

// speed isn't as important.  Otherwise, just leave this undefined.

//

// `define      OPT_SINGLE_FETCH

//

//

//

// The next several options are pipeline optimization options.  They make no

// sense in a single instruction fetch mode, hence we #ifndef them so they

// are only defined if we are in a full pipelined mode (i.e. OPT_SINGLE_FETCH

// is not defined).

//

`ifndef OPT_SINGLE_FETCH

//

//

//

// OPT_PIPELINED is the natural result and opposite of using the single 

// instruction fetch unit.  If you are not using that unit, the ZipCPU will

// be pipelined.  The option is defined here more for readability than 

// anything else, since OPT_PIPELINED makes more sense than OPT_SINGLE_FETCH,

// well ... that and it does a better job of explaining what is going on.

//

// In other words, leave this define alone--lest you break the ZipCPU.

//

`define OPT_PIPELINED

//

//

//

// OPT_TRADITIONAL_PFCACHE allows you to switch between one of two prefetch

// caches.  If enabled, a more traditional cache is implemented.  This more

// traditional cache (currently) uses many more LUTs, but it also reduces

// the stall count tremendously over the alternative hacked pipeline cache.

// (The traditional pfcache is also pipelined, whereas the pipeline cache

// implements a windowed approach to caching.)

//

// If you have the fabric to support this option, I recommend including it.

//

`define OPT_TRADITIONAL_PFCACHE

//

//

//

// OPT_EARLY_BRANCHING is an attempt to execute a BRA statement as early

// as possible, to avoid as many pipeline stalls on a branch as possible.

// It's not tremendously successful yet--BRA's still suffer stalls,

// but I intend to keep working on this approach until the number of stalls

// gets down to one or (ideally) zero.  (With the OPT_TRADITIONAL_PFCACHE, this

// gets down to a single stall cycle ...)  That way a "BRA" can be used as the

// compiler's branch prediction optimizer: BRA's barely stall, while branches

// on conditions will always suffer about 4 stall cycles or so.

//

// I recommend setting this flag, so as to turn early branching on.

//

`define OPT_EARLY_BRANCHING

//

//

//

// OPT_PIPELINED_BUS_ACCESS controls whether or not LOD/STO instructions

// can take advantaged of pipelined bus instructions.  To be eligible, the

// operations must be identical (cannot pipeline loads and stores, just loads

// only or stores only), and the addresses must either be identical or one up

// from the previous address.  Further, the load/store string must all have

// the same conditional.  This approach gains the must use, in my humble

// opinion, when saving registers to or restoring registers from the stack

// at the beginning/end of a procedure, or when doing a context swap.

//

// I recommend setting this flag, for performance reasons, especially if your

// wishbone bus can handle pipelined bus accesses.

//

`define OPT_PIPELINED_BUS_ACCESS

//

//

//

`ifdef  OPT_NEW_INSTRUCTION_SET

//

//

//

// The new instruction set also defines a set of very long instruction words.

// Well, calling them "very long" instruction words is probably a misnomer,

// although we're going to do it.  They're really 2x16-bit instructions---

// instruction words that pack two instructions into one word.  (2x14 bit

// really--'cause you need a bit to note the instruction is a 2x instruction,

// and then 3-bits for the condition codes ...)  Set OPT_VLIW to include these

// double instructions as part of the new instruction set.  These allow a single

// instruction to contain two instructions within.   These instructions are

// designed to get more code density from the instruction set, and to hopefully

// take some pain off of the performance of the pre-fetch and instruction cache.

//

// These new instructions, however, also necessitate a change in the Zip

// CPU--the Zip CPU can no longer execute instructions atomically.  It must

// now execute non-VLIW instructions, or VLIW instruction pairs, atomically. 

// This logic has been added into the ZipCPU, but it has not (yet) been

// tested thoroughly.

//

// Oh, and the assembler, the debugger, and the object file dumper, and the

// simulator all need to be updated as well ....

//

`define OPT_VLIW

//

//

`endif // OPT_NEW_INSTRUCTION_SET

//

//

`endif  // OPT_SINGLE_FETCH

//

//

//

// Now let's talk about peripherals for a moment.  These next two defines

// control whether the DMA controller is included in the Zip System, and

// whether or not the 8 accounting timers are also included.  Set these to

// include the respective peripherals, comment them out not to.

//

`define INCLUDE_DMA_CONTROLLER

`define INCLUDE_ACCOUNTING_COUNTERS

//

//

// `define      DEBUG_SCOPE

//

`endif  // CPUDEFS_H