Gopher2600/hardware/memory/cartridge/mapper_cbs.go

// This file is part of Gopher2600.
//
// Gopher2600 is free software: 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.
//
// Gopher2600 is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY 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 Gopher2600.  If not, see <https://www.gnu.org/licenses/>.

package cartridge

import (
	"fmt"

	"github.com/jetsetilly/gopher2600/environment"
	"github.com/jetsetilly/gopher2600/hardware/memory/cartridge/mapper"
	"github.com/jetsetilly/gopher2600/hardware/memory/memorymap"
)

// from bankswitch_sizes.txt:
//
// 12K:
//
//	-FA: Used only by CBS.  Similar to F8, except you have three 4K banks
//	instead of two.  You select the desired bank via 1FF8, 1FF9, and 1FFA.
//	These carts also have 256 bytes of RAM mapped in at 1000-11FF.  1000-10FF
//	is the write port while 1100-11FF is the read port.
//
// cartridges:
//   - Omega Race
//   - Mountain King
//   - Tunnel Runner
//   - Noice (scene demo)
//
// US patent 4,485,457A describes the format in detail:
//
// https://patents.google.com/patent/US4485457A/en
type cbs struct {
	env *environment.Environment

	mappingID string

	// cbs cartridges have 3 banks of 4096 bytes
	bankSize int
	banks    [][]uint8

	// rewindable state
	state *cbsState
}

func newCBS(env *environment.Environment, data []byte) (mapper.CartMapper, error) {
	cart := &cbs{
		env:       env,
		mappingID: "FA",
		bankSize:  4096,
		state:     newCbsState(),
	}

	if len(data) != cart.bankSize*cart.NumBanks() {
		return nil, fmt.Errorf("FA: wrong number of bytes in the cartridge data")
	}

	cart.banks = make([][]uint8, cart.NumBanks())

	for k := 0; k < cart.NumBanks(); k++ {
		cart.banks[k] = make([]uint8, cart.bankSize)
		offset := k * cart.bankSize
		copy(cart.banks[k], data[offset:offset+cart.bankSize])
	}

	return cart, nil
}

// MappedBanks implements the mapper.CartMapper interface.
func (cart *cbs) MappedBanks() string {
	return fmt.Sprintf("Bank: %d", cart.state.bank)
}

// ID implements the mapper.CartMapper interface.
func (cart *cbs) ID() string {
	return cart.mappingID
}

// Snapshot implements the mapper.CartMapper interface.
func (cart *cbs) Snapshot() mapper.CartMapper {
	n := *cart
	n.state = cart.state.Snapshot()
	return &n
}

// Plumb implements the mapper.CartMapper interface.
func (cart *cbs) Plumb(env *environment.Environment) {
	cart.env = env
}

// Reset implements the mapper.CartMapper interface.
func (cart *cbs) Reset() {
	for i := range cart.state.ram {
		if cart.env.Prefs.RandomState.Get().(bool) {
			cart.state.ram[i] = uint8(cart.env.Random.NoRewind(0xff))
		} else {
			cart.state.ram[i] = 0
		}
	}

	cart.state.bank = len(cart.banks) - 1
}

// Access implements the mapper.CartMapper interface.
func (cart *cbs) Access(addr uint16, _ bool) (uint8, uint8, error) {
	if addr <= 0x00ff {
		return 0, 0, nil
	}
	if addr >= 0x0100 && addr <= 0x01ff {
		return cart.state.ram[addr-0x100], mapper.CartDrivenPins, nil
	}

	return cart.banks[cart.state.bank][addr], mapper.CartDrivenPins, nil
}

// AccessVolatile implements the mapper.CartMapper interface.
func (cart *cbs) AccessVolatile(addr uint16, data uint8, poke bool) error {
	cart.bankswitch(addr, data)

	if addr <= 0x00ff {
		cart.state.ram[addr] = data
		return nil
	}

	if poke {
		cart.banks[cart.state.bank][addr] = data
		return nil
	}

	return nil
}

// bankswitch on hotspot access.
func (cart *cbs) bankswitch(addr uint16, data uint8) bool {
	// bank switching happens only if the data bus low bit is set to one.
	// From the patent:
	//
	// "Address lines A0 through A12, and data lines A0 through A7 are
	// connected to page select decode logic 82, the page select decode
	// logic 82 being responsive to the combination of a particular address
	// and predetermined data information. In the present embodiment, the
	// logic is responsive to the data line D0 being in a high state (i.e.,
	// the signal on data line D0 is a binary "1") and the other data lines
	// inactive."

	// the data bus condition is required in hardware, according to the patent,
	// but it's not strictly required in emulation. this is because the
	// emulation doesn't have the same limitation as the hardware in this
	// respect.
	//
	// none-the-less the condition is replicated here to protect against the
	// theoretical instance of someone developing a new CBS ROM and
	// inadvertently accepting a bankswitch event that couldn't happen in
	// hardware.

	if data&0x01 == 0x01 {
		if addr == 0x0ff8 {
			cart.state.bank = 0
		} else if addr == 0x0ff9 {
			cart.state.bank = 1
		} else if addr == 0x0ffa {
			cart.state.bank = 2
		} else {
			return false
		}
	}

	return true
}

// NumBanks implements the mapper.CartMapper interface.
func (cart *cbs) NumBanks() int {
	return 3
}

// GetBank implements the mapper.CartMapper interface.
func (cart *cbs) GetBank(addr uint16) mapper.BankInfo {
	// cbs cartridges are like atari cartridges in that the entire address
	// space points to the selected bank
	return mapper.BankInfo{Number: cart.state.bank, IsRAM: addr <= 0x00ff}
}

// Patch implements the mapper.CartPatchable interface
func (cart *cbs) Patch(offset int, data uint8) error {
	if offset >= cart.bankSize*len(cart.banks) {
		return fmt.Errorf("FA: patch offset too high (%d)", offset)
	}

	bank := offset / cart.bankSize
	offset %= cart.bankSize
	cart.banks[bank][offset] = data
	return nil
}

// AccessPassive implements the mapper.CartMapper interface.
func (cart *cbs) AccessPassive(addr uint16, data uint8) error {
	return nil
}

// Step implements the mapper.CartMapper interface.
func (cart *cbs) Step(_ float32) {
}

// GetRAM implements the mapper.CartRAMBus interface.
func (cart *cbs) GetRAM() []mapper.CartRAM {
	r := make([]mapper.CartRAM, 1)
	r[0] = mapper.CartRAM{
		Label:  "CBS+RAM",
		Origin: 0x1080,
		Data:   make([]uint8, len(cart.state.ram)),
		Mapped: true,
	}
	copy(r[0].Data, cart.state.ram)
	return r
}

// PutRAM implements the mapper.CartRAMBus interface.
func (cart *cbs) PutRAM(_ int, idx int, data uint8) {
	cart.state.ram[idx] = data
}

// IterateBank implements the mapper.CartMapper interface.
func (cart *cbs) CopyBanks() []mapper.BankContent {
	c := make([]mapper.BankContent, len(cart.banks))
	for b := 0; b < len(cart.banks); b++ {
		c[b] = mapper.BankContent{Number: b,
			Data:    cart.banks[b],
			Origins: []uint16{memorymap.OriginCart},
		}
	}
	return c
}

// ReadHotspots implements the mapper.CartHotspotsBus interface.
func (cart *cbs) ReadHotspots() map[uint16]mapper.CartHotspotInfo {
	return map[uint16]mapper.CartHotspotInfo{
		0x1ff8: {Symbol: "BANK0", Action: mapper.HotspotBankSwitch},
		0x1ff9: {Symbol: "BANK1", Action: mapper.HotspotBankSwitch},
		0x1ffa: {Symbol: "BANK2", Action: mapper.HotspotBankSwitch},
	}
}

// WriteHotspots implements the mapper.CartHotspotsBus interface.
func (cart *cbs) WriteHotspots() map[uint16]mapper.CartHotspotInfo {
	return cart.ReadHotspots()
}

// rewindable state for the CBS cartridge.
type cbsState struct {
	// identifies the currently selected bank
	bank int

	// CBS cartridges had internal RAM very similar to the Atari Superchip
	ram []uint8
}

func newCbsState() *cbsState {
	const cbsRAMsize = 256

	return &cbsState{
		ram: make([]uint8, cbsRAMsize),
	}
}

// Snapshot implements the mapper.CartMapper interface.
func (s *cbsState) Snapshot() *cbsState {
	n := *s
	n.ram = make([]uint8, len(s.ram))
	copy(n.ram, s.ram)
	return &n
}