Atari - put descriptions at the top of each mapper file
This commit is contained in:
adelikat
parent
fffe30f4f9
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e0a6ae5264
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@ -5,6 +5,25 @@ using System.Text;
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namespace BizHawk.Emulation.Consoles.Atari._2600
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{
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/*
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This is another 8K bankswitching method with two 4K banks. The rationale is that it's
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cheap and easy to implement with only a single 74HC153 or 253 dual 4:1 multiplexer.
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This multiplexer can act as a 1 bit latch AND the inverter for A12.
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To bankswitch, the following mask it used:
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A13 A0
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----------------
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0 1xxx xBxx xxxx
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Each bit corresponds to one of the 13 address lines. a 0 or 1 means that bit must be
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0 or 1 to trigger the bankswitch. x is a bit that is not concidered (it can be either
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0 or 1 and is thus a "don't care" bit).
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B is the bank we will select. sooo, accessing 0800 will select bank 0, and 0840
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will select bank 1.
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*/
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class m0840 : MapperBase
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{
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@ -5,6 +5,24 @@ using System.Text;
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namespace BizHawk.Emulation.Consoles.Atari._2600
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{
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/*
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3E (Boulderdash
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-----
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This works similar to 3F (Tigervision) above, except RAM has been added. The range of
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addresses has been restricted, too. Only 3E and 3F can be written to now.
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1000-17FF - this bank is selectable
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1800-1FFF - this bank is the last 2K of the ROM
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To select a particular 2K ROM bank, its number is poked into address 3F. Because there's
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8 bits, there's enough for 256 2K banks, or a maximum of 512K of ROM.
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Writing to 3E, however, is what's new. Writing here selects a 1K RAM bank into
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1000-17FF. The example (Boulderdash) uses 16K of RAM, however there's theoretically
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enough space for 256K of RAM. When RAM is selected, 1000-13FF is the read port while
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1400-17FF is the write port.
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*/
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class m3E : MapperBase
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{
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@ -5,6 +5,25 @@ using System.Text;
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namespace BizHawk.Emulation.Consoles.Atari._2600
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{
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/*
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3F (Tigervision)
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-----
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Traditionally, this method was used on the Tigervision games. The ROMs were all 8K, and
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there's two 2K pages in the 4K of address space. The upper bank is fixed to the last 2K
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of the ROM.
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The first 2K is selectable by writing to any location between 0000 and 003F. Yes, this
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overlaps the TIA, but this is not a big deal. You simply use the mirrors of the TIA at
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40-7F instead! To select a bank, the games write to 3Fh, because it's not implemented
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on the TIA.
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The homebrew community has decided that if 8K is good, more ROM is better! This mapper
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can support up to 512K bytes of ROM just by implementing all 8 bits on the mapper
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register, and this has been done... however I do not think 512K ROMs have been made just
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yet.
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*/
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class m3F :MapperBase
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{
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@ -5,6 +5,31 @@ using System.Text;
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namespace BizHawk.Emulation.Consoles.Atari._2600
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{
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/*
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4A50 (no name)
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-----
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Upon review, I don't think this method is terribly workable on real
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hardware. There's so many problems that I kinda gave up trying to
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count them all. Seems that this is more of a "pony" method than something
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actually usable. ("pony" referring to "I want this, and that, and that, and
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a pony too!")
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One major problem is that it specifies that memory can be read and written
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to at the same address, but this is nearly impossible to detect on a 2600
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cartridge. You'd almost have to try and figure out what opcodes are being
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run, and what cycle it's on somehow, all just by watching address and
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data bus state. Not very practical.
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The other problem is just the sheer volume of things it is supposed to do.
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There's just tons and tons of unnecessary things like attempting to detect
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BIT instructions, handling page wraps and other silly things.
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This all supposidly fit into a Xilinx XC9536XL but I am not sure how the
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chip could handle the RAM issue above at all. It almost needs to see R/W
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and M2 (clock) to be able to properly do most of the things it's doing.
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*/
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class m4A50 : MapperBase
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{
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@ -5,6 +5,20 @@ using System.Text;
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namespace BizHawk.Emulation.Consoles.Atari._2600
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{
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/*
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This was used by Commavid. It allowed for both ROM and RAM on the cartridge,
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without using bankswitching. There's 2K of ROM and 1K of RAM.
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2K of ROM is mapped at 1800-1FFF.
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1K of RAM is mapped in at 1000-17FF.
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The read port is at 1000-13FF.
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The write port is at 1400-17FF.
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Example games:
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Magicard
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*/
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class mCV: MapperBase
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{
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ByteBuffer aux_ram = new ByteBuffer(1024);
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@ -5,6 +5,27 @@ using System.Text;
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namespace BizHawk.Emulation.Consoles.Atari._2600
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{
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/*
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E0 (Parker Bros)
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-----
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Parker Brothers used this, and it was used on one other game (Tooth Protectors). It
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uses 8K of ROM and can map 1K sections of it.
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This mapper has 4 1K banks of ROM in the address space. The address space is broken up
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into the following locations:
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1000-13FF : To select a 1K ROM bank here, access 1FE0-1FE7 (1FE0 = select first 1K, etc)
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1400-17FF : To select a 1K ROM bank, access 1FE8-1FEF
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1800-1BFF : To select a 1K ROM bank, access 1FF0-1FF7
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1C00-1FFF : This is fixed to the last 1K ROM bank of the 8K
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Like F8, F6, etc. accessing one of the locations indicated will perform the switch.
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Example Games:
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Frogger II - Threedeep! (1983) (Parker Bros)
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*/
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class mE0 : MapperBase
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{
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int toggle1 = 0;
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@ -5,6 +5,16 @@ using System.Text;
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namespace BizHawk.Emulation.Consoles.Atari._2600
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{
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/*
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EF (no name?)
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-----
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This is a fairly simple method that allows for up to 64K of ROM, using 16 4K banks.
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It works similar to F8, F6, etc. Only the addresses to perform the switch is
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1FE0-1FEF. Accessing one of these will select the desired bank. 1FE0 = bank 0,
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1FE1 = bank 1, etc.
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*/
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class mEF : MapperBase
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{
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@ -6,7 +6,7 @@ using System.Text;
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namespace BizHawk.Emulation.Consoles.Atari._2600
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{
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/*
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* F0 (Megaboy)
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F0 (Megaboy)
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-----
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This was used on one game, "megaboy".. Some kind of educational cartridge. It supports
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@ -5,6 +5,14 @@ using System.Text;
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namespace BizHawk.Emulation.Consoles.Atari._2600
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{
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/*
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F4 (Atari style 32K)
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-----
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Again, this works like F8 and F6 except now there's 8 4K banks. Selection is performed
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by accessing 1FF4 through 1FFB.
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*/
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class mF4 :MapperBase
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{
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int toggle = 0;
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@ -5,6 +5,15 @@ using System.Text;
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namespace BizHawk.Emulation.Consoles.Atari._2600
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{
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/*
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F6 (Atari style 16K)
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-----
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This is a fairly standard 16K bankswitching method. It works like F8, except there's
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four 4K banks of ROM, selected by accessing 1FF6 through 1FF9. These sequentially
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select one of the 4 banks. i.e. 1FF6 selects bank 0, 1FF7 selects bank 1, etc.
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*/
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class mF6 : MapperBase
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{
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int toggle = 0;
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@ -8,6 +8,23 @@ namespace BizHawk
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{
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partial class Atari2600
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{
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/*
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F8 (Atari style 8K)
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-----
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This is the fairly standard way 8K of cartridge ROM was implemented. There are two
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4K ROM banks, which get mapped into the 4K of cartridge space. Accessing 1FF8 or
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1FF9 selects one of the two 4K banks. When one of these two addresses are accessed,
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the banks switch spontaniously.
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ANY kind of access will trigger the switching- reading or writing. Usually games use
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LDA or BIT on 1FF8/1FF9 to perform the switch.
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When the switch occurs, the entire 4K ROM bank switches, including the code that is
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reading the 1FF8/1FF9 location. Usually, games put a small stub of code in BOTH banks
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so when the switch occurs, the code won't crash.
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*/
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class mF8 : MapperBase
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{
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int toggle = 0;
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@ -5,6 +5,61 @@ using System.Text;
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namespace BizHawk.Emulation.Consoles.Atari._2600
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{
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/*
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FE (Activision special)
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-----
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Activision used this method on only three games: Decathlon, Robot Tank, and the
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prototype Thwocker. This mapper is one of the more interesting ones in that it uses
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stack access to select banks. It is composed of two 4K banks, similar to F8.
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Unlike F8, however, switching occurs when the stack is accessed.
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This mapper allows for "automatic" bankswitching to occur, using JSR and RTS. The
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addresses for all JSRs and RTS' are either Fxxx or Dxxx, and the mapper uses this to
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figure out which bank it should be going to.
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The cycles of a JSR are as such:
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1: opcode fetch
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2: fetch low byte of address
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3: read 100,s : garbage fetch
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4: write 100,s : PCH, decrement S
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5: write 100,s : PCL, decrement S
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6: fetch high byte of address
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The cycles of an RTS are as such:
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1: opcode fetch
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2: fetch next opcode (and throw it away)
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3: read 100,S : increment S
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4: read 100,S : pull PCL from stack, increment S
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5: read 100,S : pull PCH from stack
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The chip can determine what instruction is being executed by watching the data and
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address bus.
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It watches for 20 (JSR) and 60 (RTS), and accesses to 100-1ff:
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(opcode cycles)
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20 (opcode)
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add low (new add low)
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stack (garbage read)
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stack (push PCH)
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stack (push PCL)
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add high (new add hi) : latch D5. This is the NEW bank we need to be in.
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60 (opcode)
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xx (garbage fetch)
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stack
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stack (pull PCL)
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stack (pull PCH) : latch D5. This is the NEW bank we need to be in.
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Using emulators or similar there is a large cheat that can be used. A13 can be used
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to simply select which 8K bank to be in.
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*/
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class mFE : MapperBase
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{
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@ -5,6 +5,63 @@ using System.Text;
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namespace BizHawk.Emulation.Consoles.Atari._2600
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{
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/*
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MC (Megacart)
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-----
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This is the mapper for the "Chris Wilkson's Megacart".
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Only four addresses are used to bankswitch on this one.
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Up to 128K of ROM and 64K of RAM can be accessed.
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1000-13FF is selected by address 3C
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1400-17FF is selected by address 3D
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1800-1BFF is selected by address 3E
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1C00-1FFF is selected by address 3F
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The value written determines what will be selected:
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00-7F written will select one of the 128 1K ROM banks
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80-FF written will select one of the 128 512 byte RAM banks
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When a RAM bank is selected, the lower 512 bytes is the write port, while
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the upper 512 bytes is the read port.
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On accessing address FFFC or FFFD, the last 1K bank points to the last bank in ROM,
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to allow for system initialization. Jumping out of the last bank disables this.
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It's debatable how easy this system would be to implement on a real system.
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Detecting when to disable the last bank fixing is difficult. The documentation
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says:
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"
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Megacart Specification, Rev1.1
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(c) 1997 Chris Wilkson
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cwilkson@mit.edu
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Because the console's memory is randomized at powerup, there is no way to
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predict the data initially contained in the "hot addresses". Therefore,
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hardware will force slot 3 to always point to ROM block $FF immediately
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after any read or write to the RESET vector at $FFFC-$FFFD. Block $FF
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must contain code to initialize the 4 memory slots to point to the desired
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physical memory blocks before any other code can be executed. After program
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execution jumps out of the boot code, the hardware will release slot 3 and
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it will function just like any other slot.
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"
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Unfortunately, there's not an easy way to detect this. Just watching the address
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bus won't work easily: Writing anywhere outside the bank 1C00-1FFF (i.e. bank
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registers, RAM, TIA registers) will cause the switching to revert bank 3, crashing
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the system.
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The only way I can see it working is to disregard any access to addresses 3C-3F.
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Emulators have it easier: they can simply watch the program counter, vs. the
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address bus. An actual system doesn't have that luxury, unfortunately, so it must
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disregard accesses to 3C-3F instead.
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*/
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class mMC : MapperBase
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{
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@ -5,6 +5,37 @@ using System.Text;
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namespace BizHawk.Emulation.Consoles.Atari._2600
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{
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/*
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X07 (Atariage)
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-----
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Apparently, this was only used on one cart: Stella's Stocking.
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Similar to EF, there are 16 4K banks, for a total of up to 64K of ROM.
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The addresses to select banks is below the ROM area, however.
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The following TWO masks are used:
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A13 A0
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----------------
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0 1xxx nnnn 1101
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This means the address 80B selects bank 0, 81B selects bank 1, etc.
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In addition to this, there is another way:
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A13 A0
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----------------
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0 0xxx 0nxx xxxx
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This is somewhat special purpose: Accessing here does nothing, unless one of the
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last two banks are selected (banks 14 or 15). In that case, the new bank is:
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111n i.e. accessing 0000 will select bank 14 (Eh, 1110b) while accessing 0040
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will select bank 15 (Fh, 1111b). This allows for bankswitching by accessing
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TIA registers at 00-3F or 40-7F without incurring any overhead.
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*/
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class mX07 : MapperBase
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{
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