addresses

Documentation

Overview

Package addresses contains all information about VCS addresses and registers, including canonical symbols for read and write addresses and registers. These symbols are used by the symbols package to create the symbol table for an inserted cartridge. They will be supplemented by cartridge specific symbols if a symbols file is available (see symbols package for details).

In addition to the canonical symbol maps, there are two sparse arrays Read and Write, created from the canonical maps at run time. These arrays are used by the emulator for speed purposes - accessing a map although very convnient, is noticeably slower than accessing a sparse array. There is probably no need to use this arrays outside of the emulation code.

Index

• Constants
• Variables
• type ChipRegister

Constants

const IRQ = uint16(0xfffe)

IRQ is the address where the interrupt address is stored.

const Reset = uint16(0xfffc)

Reset is the address where the reset address is stored - used by VCS.Reset() and Disassembly module.

Variables

var DataMasks = []uint8{
	0b11000000,
	0b11000000,
	0b11000000,
	0b11000000,
	0b11000000,
	0b11000000,

	0b11000000,

	0b11000000,
	0b10000000,
	0b10000000,
	0b10000000,
	0b10000000,
	0b10000000,
	0b10000000,

	0b11000000,
	0b11000000,
}

DataMasks are applied to data read by the CPU from lowest 16 addresses. This requirement is a consequence of how the address/data bus works in the VCS.

For example, if the CPU wants to read the contents of the CXM1P register, it can use the address 0x0d to do so.

LDA 0x01

If there are no collisions (between missile 1 and either player, in this case) than the value of the most significant bits are zero. The lower six bits are not part of the CXM1P register and are left undefined by the TIA when the data is put on the bus. The lower bits of the LDA operation are in fact "left over" from the address. In our example, the lowest six bits are

0bxx000001

meaning the the returned data is in fact 0x01 and not 0x00, as you might expect. Things get interesting when we use mirrored addresses. If instead of 0x01 we used the mirror address 0x11, the lowest six bits are:

0bxx01001

meaning that the returned value is 0x11 and not (again, as you might expect) 0x00 or even 0x01.

So what happens if there is sprite collision information in the register? Meaning that the top bits are not necessarily zero. Let's say there is a collusion between missile 1 and player 0, the data before masking will be

0b01000000

If we used address 0x11 to load this value, we would in fact, get this pattern (0x51 in hex):

0b01010001

Now, if all ROMs read and interpreted chip registers only as they're supposed to (defails in the 2600 programmer's guide) then none of this would matter but some ROMs do make use of the extra bits, and so we must account for it in emulation.

It's worth noting that the above is implicitly talking about zero-page addressing; but masking also occurs with regular two-byte addressing. The key to understanding is that the masking is applied to the most recent byte of the address to be put on the address bus*. In all cases, this is the most-significant byte. So, if the requested address is 0x171, the bit pattern for the address is:

0x0000000101110001

the most significant byte in this pattern is 0x00000001 and so the data retreived is AND-ed with that. The mapped address for 0x171 incidentally, is 0x01, which is the CXM1P register also used in the examples above.

var RIOTReadSymbols = map[uint16]string{

	0x0280: "SWCHA",
	0x0281: "SWACNT",
	0x0282: "SWCHB",
	0x0283: "SWBCNT",
	0x0284: "INTIM",
	0x0285: "TIMINT",
}

RIOTReadSymbols indexes all RIOT read symbols by normalised address.

var RIOTWriteSymbols = map[uint16]string{
	0x0280: "SWCHA",
	0x0281: "SWACNT",
	0x0294: "TIM1T",
	0x0295: "TIM8T",
	0x0296: "TIM64T",
	0x0297: "T1024T",
}

RIOTWriteSymbols indexes all RIOT write symbols by normalised address.

var Read []string

Read is a sparse array containing the canonical labels for VCS read addresses. If the address indexes as empty string then the address is not reabable.

var ReadAddress = map[string]uint16{}

ReadAddress indexes all VCS read addresses by canonical symbol.

var ReadSymbols = map[uint16]string{}

ReadSymbols indexes all VCS read symbols by normalised address.

var TIAReadSymbols = map[uint16]string{

	0x00: "CXM0P",
	0x01: "CXM1P",
	0x02: "CXP0FB",
	0x03: "CXP1FB",
	0x04: "CXM0FB",
	0x05: "CXM1FB",
	0x06: "CXBLPF",
	0x07: "CXPPMM",
	0x08: "INPT0",
	0x09: "INPT1",
	0x0a: "INPT2",
	0x0b: "INPT3",
	0x0c: "INPT4",
	0x0d: "INPT5",
}

TIAReadSymbols indexes all TIA read symbols by normalised address.

var TIAWriteSymbols = map[uint16]string{
	0x00: "VSYNC",
	0x01: "VBLANK",
	0x02: "WSYNC",
	0x03: "RSYNC",
	0x04: "NUSIZ0",
	0x05: "NUSIZ1",
	0x06: "COLUP0",
	0x07: "COLUP1",
	0x08: "COLUPF",
	0x09: "COLUBK",
	0x0a: "CTRLPF",
	0x0b: "REFP0",
	0x0c: "REFP1",
	0x0d: "PF0",
	0x0e: "PF1",
	0x0f: "PF2",
	0x10: "RESP0",
	0x11: "RESP1",
	0x12: "RESM0",
	0x13: "RESM1",
	0x14: "RESBL",
	0x15: "AUDC0",
	0x16: "AUDC1",
	0x17: "AUDF0",
	0x18: "AUDF1",
	0x19: "AUDV0",
	0x1a: "AUDV1",
	0x1b: "GRP0",
	0x1c: "GRP1",
	0x1d: "ENAM0",
	0x1e: "ENAM1",
	0x1f: "ENABL",
	0x20: "HMP0",
	0x21: "HMP1",
	0x22: "HMM0",
	0x23: "HMM1",
	0x24: "HMBL",
	0x25: "VDELP0",
	0x26: "VDELP1",
	0x27: "VDELBL",
	0x28: "RESMP0",
	0x29: "RESMP1",
	0x2A: "HMOVE",
	0x2B: "HMCLR",
	0x2C: "CXCLR",
}

TIAWriteSymbols indexes all TIA write symbols by normalised address.

var Write []string

Write is a sparse array containing the canonical labels for VCS write addresses. If the address indexes an empty string then the address is not writable.

var WriteAddress = map[string]uint16{}

WriteAddress indexes all VCS write addresses by canonical symbol.

var WriteSymbols = map[uint16]string{}

WriteSymbols indexes all VCS write symbols by normalised address.

Types

type ChipRegister

type ChipRegister int

ChipRegister specifies the offset of a chip register in the chip memory areas. It is used in contexts where a register is required, as opposed to an address.

const (
	CXM0P ChipRegister = iota
	CXM1P
	CXP0FB
	CXP1FB
	CXM0FB
	CXM1FB
	CXBLPF
	CXPPMM
	INPT0
	INPT1
	INPT2
	INPT3
	INPT4
	INPT5
)

TIA registers

These value are used by the emulator to specify known addresses. For example, when writing collision information we know we need the CXM0P register. these named values make the code more readable

Values are enumerated from 0; value is added to the origin address of the TIA in ChipBus.ChipWrite implementation.

const (
	SWCHA ChipRegister = iota
	SWACNT
	SWCHB
	SWBCNT
	INTIM
	TIMINT
)

RIOT registers

These value are used by the emulator to specify known addresses. For example, the timer updates itself every cycle and stores time remaining value in the INTIM register.

Values are enumerated from 0; value is added to the origin address of the TIA in ChipBus.ChipWrite implementation.