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) 2015-2016, 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.

//

// You should have received a copy of the GNU General Public License along

// with this program.  (It's in the $(ROOT)/doc directory.  Run make with no

// target there if the PDF file isn't present.)  If not, see

// <http://www.gnu.org/licenses/> for a copy.

//

// 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 so highly, that I'm likely going to remove

// the option to turn this off in future versions of this CPU.

//

`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.

//

// Either not defining this value, or defining it to zero will disable the

// hardware multiply.  A value of '1' will cause the multiply to occurr in one

// clock cycle only--often at the expense of the rest of the CPUs speed.

// A value of 2 will cause the multiply to have a single delay cycle, 3 will

// have two delay cycles, and 4 (or more) will have 3 delay cycles.

//

//

`define OPT_MULTIPLY    3

//

//

//

// 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_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

//

//

// OPT_DOUBLE_FETCH is an alternative to OPT_SINGLE_FETCH.  It is designed to

// increase performance primarily when using an instruction memory which has

// one cost for a random access, and a second (lower) cost for sequential

// access.  The driving example behind this implementation was flash memory

// with 34 clocks for an initial access and 16 clocks for any subsequent access,

// but SDRAM memory with 27 clocks for an initial access and 1 clock for a

// subsequent access is also a good example.  Even block RAM might be a good

// example, if there were any bus delays in getting to the RAM device.  Using

// OPT_DOUBLE_FETCH also increases the pipeline speed, as it allows CIS

// instructions and therefore partial pipelining.  (No work is done to resolve

// pipeline conflicts past the decode stage, as is the case with full pipeline

// mode.

//

// Do not define OPT_DOUBLE_FETCH if you wish to fully pipeline the CPU.  Do

// not define both OPT_DOUBLE_FETCH and OPT_SINGLE_FETCH (the ifndef below

// should prevent that).

//

//

`ifndef OPT_SINGLE_FETCH

// `define      OPT_DOUBLE_FETCH

`endif

//

//

//

// The ZipCPU ISA defines an optional compressed instruction set (CIS)

// complement.  This compressed instruction format allows two instructions to

// be packed into the same instruction word.  Some instructions can be so

// compressed, although not all.  Compressed instructions take the same time to

// complete--they are just compressed within memory to spare troubles with the

// prefetch.  Set OPT_CIS to include these compressed instructions as part of

// the instruction set.

//

`define OPT_CIS         // COST: about 87 LUTs

//

//

//

//

// 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.

// With the OPT_TRADITIONAL_PFCACHE, BRA instructions cost only a single

// extra stall cycle, while LJMP instructions cost two (assuming the target is

// in the cache).  Indeed, the result is that a BRA instruction can be used as

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

// conditional branches will always suffer about 4 stall cycles or so.

//

// I recommend setting this flag, so as to turn early branching on---if you

// have the LUTs available to afford it.

//

`define OPT_EARLY_BRANCHING

//

//

//

//

// 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

`ifndef OPT_DOUBLE_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_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

//

//

//

//

//

`endif  // OPT_DOUBLE_FETCH

`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