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The Intellicart is a recently-produced RAM-based cartridge for the Intellivision, designed and sold by Chad Schell. It provides 64Kx16-bit of memory for games. Through a clever bankswitching scheme, modern software can use all 64K words. It is also capable of playing nearly all existing games. The few it cannot play without modification require 8-bit RAM or ECS-style bank switching.
Memory Model: The Basics
Because the Intellivision's memory map is only 64K-words large, and because the Intellicart offers 64K-words worth of storage, it is neither possible nor desirable to directly map the Intellicart's RAM into the Intellivision's address space. There are already many devices and ROMs in the Intellivision address space that must be dealt with.
The Intellicart solves the problem by keeping the two memory spaces separate. Thus, there is the Intellivision address space and the Intellicart address space. The Intellicart then maps ranges of Intellivision addresses to its own private Intellicart addresses.
Ranges of addresses can be marked with three independent attributes:
- Readable: Addresses in this range will respond to reads
- Writeable: Addresses in this range will respond to writes
- Bankswitched: Addresses in this range may point to anywhere in the Intellicart address map.
Marking a range as Readable but not Writeable simulates ROM. Marking a range as both Readable and Writeable simulates RAM. Marking a range as Writeable-only simulates WOM (Write-Only Memory). WOM is not very useful by itself, but may be useful when combined with bankswitched memory, which is discussed below. The Intellicart will not respond on range which is neither readable nor writeable, thus allowing it to not interfere with other devices in the system.
Some emulators (such as jzIntv) also support an additional attribute which the Intellicart presently does not. This one is for support of 8-bit RAM emulation:
- Narrow: Ignore writes to the upper byte of writeable areas.
These memory attributes may be specified with a 2K-word granularity. That is, the Intellicart carves the address space as seen by the Intellivision into 32 2K-word ranges, and each range may be marked separately with these attributes. This allows specifying ranges of RAM and ROM in a cartridge with reasonable flexibility.
The Intellicart also supports "fine-address mapping." This is intended to get around "potholes" in the memory map. With this feature, a given 2K range may be "trimmed" at one or both ends in 256-word chunks. A 2K-word address range may be trimmed from both ends, but never in the middle. The trimmed portion of the range does not respond to memory accesses, and so the Intellicart won't interfere with devices in that range.
For instance, consider the 2K range $0800 - $0FFF. The Intellivoice maps its peripheral expansion bus at $0700 - $0CFF. (See Memory Map for details.) If we wish to map memory here, we need to avoid overlapping the Intellivoice. Fine-address mapping allows us to map memory starting at $0D00 rather than $0800 to avoid the hole.
Memory Model: Bank-Switching
By default, ranges of mapped addresses provide a direct map between the Intellivision and Intellicart address space. That is, there is an exact 1-to-1 correspondence between Intellivision and Intellicart addresses. For instance, $5000 in the Intellivision address space becomes $5000 in the Intellicart address space. For most programs, that is sufficient.
For programs that wish to use the entire Intellicart address map, or wish to employ advanced functionality, the Intellicart provides bank-switching support.
Bank-switching allows a given 2K-word range to point anywhere in the Intellicart address space with 256-word granularity. There are no restrictions whatsoever on where a given 2K range maps to. Multiple ranges may point to the same Intellicart memory -- they are controlled independently of each other.
The following diagrams attempt to illustrate how direct-mapping compares to bank-switching. (ASCII ART alert!)
- Direct mapped range $6000 - $67FF:
Intellivision Intellicart $FFFF +-----------+ +----------+ $FFFF | | | | | | | | $67FF |___________|.....................|__________| $67FF | | DIRECT MAP | | $6000 |___________|.....................|__________| $6000 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | $0000 +-----------+ +----------+
- Bankswitched range $6000 - $67FF with $0000 - $07FF currently selected:
Intellivision Intellicart $FFFF +-----------+ +----------+ $FFFF | | | | | | | | $67FF |___________| . | | | | B'. | | $6000 |___________| . A'. | | | | '. N'. | | | | '. K'. | | | | '. S'. | | | | '. W'. | | | | '. I'. | | | | '. T'. | | | | '. C'. | | | | '. H'|__________| $07FF | | '. | | $0000 |___________| '|__________| $0000
- Bankswitched range $6000 - $67FF with $3E00 - $45FF currently selected:
Intellivision Intellicart $FFFF +-----------+ +----------+ $FFFF | | | | | | | | $67FF |___________|.__ | | | | '-.__ | | $6000 |___________|._ '-.__ | | | | '--._ '-.__ | | | | '--._ '-.|__________| $45FF | | '--._ | | | | '--.|__________| $3E00 | | B A N K S W I T C H | | | | | | | | | | | | | | | | | | $0000 |___________| |__________| $0000
- Direct map range $5000 - $57FF, and bankswitched range $6000 - $67FF with $5000 - $57FF currently selected:
Intellivision Intellicart $FFFF +-----------+ +----------+ $FFFF | | | | | | | | $67FF |___________|.__ | | | | '-.__ | | $6000 |___________|._ '-.__ | | | | '--._ '-.__ | | $57FF |___________|.......'--._......'-.|__________| $57FF | | '--._ | | $5000 |___________|.................'--.|__________| $5000 | | | | | | | | | | | | | | | | | | | | $0000 |___________| |__________| $0000
Notice how in the last example, we have two ranges, one of which is bank-switched and one of which is not, both looking at the same memory. This is perfectly legal and valid. The ranges at $6000 - $67FF and $5000 - $57FF may have completely different attributes. For instance $6000 - $67FF may be writeable, and $5000 - $57FF may be read only. If the CPU writes via $6000 - $67FF to $5000 - $57FF in the Intellicart address map, it'll change what the CPU sees via the direct map at $5000 - $57FF.
Indeed, the ECScable uses just this sort of trick for loading games. It maps $5000 - $6FFF, $8000 - $BFFF, $D000 - $DFFF and $F000 - $FFFF as read-only. It then maps a bank-switched writeable range at $0E00 - $0FFF, and uses that window to fill memory behind the read-only ranges when a game is uploaded.
As noted in the example above, bank selection occurs with 256-word granularity. The next section discusses how that works.
The Intellicart's bank-switching is controlled by a series of write-only registers in lower memory. These registers live at $0040 - $005F. Only pages that are marked as bank-switchable will be affected by writes to these registers:
Address Range Modified Address Range Modified --------- ---------------- --------- ---------------- $0040 $0000 - $07FF $0050 $0800 - $0FFF $0041 $1000 - $17FF $0051 $1800 - $1FFF $0042 $2000 - $27FF $0052 $2800 - $2FFF $0043 $3000 - $37FF $0053 $3800 - $3FFF $0044 $4000 - $47FF $0054 $4800 - $4FFF $0045 $5000 - $57FF $0055 $5800 - $5FFF $0046 $6000 - $67FF $0056 $6800 - $6FFF $0047 $7000 - $77FF $0057 $7800 - $7FFF $0048 $8000 - $87FF $0058 $8800 - $8FFF $0049 $9000 - $97FF $0059 $9800 - $9FFF $004A $A000 - $A7FF $005A $A800 - $AFFF $004B $B000 - $B7FF $005B $B800 - $BFFF $004C $C000 - $C7FF $005C $C800 - $CFFF $004D $D000 - $D7FF $005D $D800 - $DFFF $004E $E000 - $E7FF $005E $E800 - $EFFF $004F $F000 - $F7FF $005F $F800 - $FFFF
Each register accepts an 8-bit value. (The upper 8 bits of the value written are ignored.) The lower 8 bits of the value written specify the upper 8 bits of the target address for that 2K range. The target address is combined with the CPU address in this manner:
target_addr = bank_select << 8; icart_addr = (intv_addr & 0x07FF) + target_addr;
That is, the bank-switching process adds a value to the upper 8 bits of the Intellivision address, *after* bits 11 through 15 have been cleared. This might seem confusing at first, so let's consider some examples.
Suppose we mark $6000 - $67FF as bank-switched, and we write $38 to that range's bank-switch register at location $0046. The CPU then makes an access to location $6123. The following steps are performed:
- The upper 5 bits of the CPU address are cleared. This leaves $0123.
- The bank-select register value is shifted left 8. This gives us $3800.
- The two values are added, yielding the final Intellicart addr: $3923.
Now suppose we write $3F to location $0046. The CPU accesses $6123 again.
- The upper 5 bits of the CPU address are cleared. This leaves $0123.
- The bank-select register value is shifted left 8. This gives us $3F00.
- The two values are added, yielding the final Intellicart addr: $4023.
Essentially, the byte written to the bank-select register specifies the starting address (to a 256-word granularity) that the given 2K-word range will map to. When we wrote $38 to location $0046, the range $6000 - $67FF in the Intellivision's address space mapped to $3800 - $3FFF in the Intellicart's address space. When we wrote $3F to location $0046, the Intellivision range $6000 - $67FF mapped to the Intellicart range $3F00 - $46FF.
NOTE: The bank-switch registers have an UNDEFINED STATE when your program starts. Therefore, do NOT place your program's startup code in a bank-switched section. This also means you should NOT map $4800, $5000 or $7000 as bank-switchable addresses, as you may confuse the EXEC boot routines. Also, make sure to set your bank selectors to known values by writing to the corresponding bank-switch registers BEFORE reading and writing the bank-switched areas.
Assembler Support in AS1600
The assembler supports the Intellicart by allowing the programmer to specify memory attributes on ranges that are being assembled. These attributes will get reflected directly in the .ROM file the assembler generates. (Or, if you generate a BIN+CFG, these will be reflected in the .CFG file.)
The assembler also supports the Intellicart by allowing the programmer to specify an Intellicart loading address that is separate from the Intellivision symbol address. This is useful for tables and/or functions that will be bank-selected between. See "examples/bankdemo" for an example.
Memory attributes are specified in three ways: Implicitly, explicitly with an ORG directive, and explicitly with a MEMATTR directive. (Please consult as1600 for additional documentation.)
By default, the assembler marks most assembled regions as "readable." The programmer may override this three ways:
- Specify an Intellicart load address that differs from the Intellivision address in an ORG directive. For example: "ORG $5000, $1000". The assembler will default to "no attribute" for code assembled following such an ORG statement.
- Specify a mode-string on an ORG statement. The mode will be applied to the given range of Intellicart addresses (not Intellivision!). For example: 'ORG $D000, $D000, "+RW"' This tells the assembler to set the Read and Write attributes for anything assembled after that ORG statement (until the next ORG is encountered).
- Use MEMATTR to modify the memory attributes on a range. You can specify either a delta-change or an absolute change. For example: 'MEMATTR $D000, $D7FF, "+RWB"' This marks the range $D000 - $D7FF as readable, writeable, and bankswitched.
Other Tools: ROM2BIN, BIN2ROM
SDK-1600 also provides two other utilities to support the Intellicart.
BIN2ROM takes a BIN and CFG file, and generates a file in .ROM format. This format is the exact format that is used for communication with the Intellicart. The following sections are recognized by BIN2ROM:
; Load word offsets $xxxx through $yyyy in BIN to Intellicart address ; $zzzz, and mark that range as direct-mapped readable in the address map: [mapping] $xxxx - $yyyy = $zzzz ; Load word offsets $xxxx through $yyyy in BIN to Intellicart address ; $zzzz. Makes no changes to the memory attributes. [preload] $xxxx - $yyyy = $zzzz ; Mark range $xxxx - $yyyy of Intellivision addrs as 8-bit or 16-bit RAM ; or WOM (write-only memory). Note that the Intellicart does not ; support 8-bit RAM/WOM, but some emulators that read the format do. ; Note: INTVPC and the Intellicart loader do not recognize WOM. [memattr] $xxxx - $yyyy = RAM 8 $xxxx - $yyyy = RAM 16 $xxxx - $yyyy = WOM 8 $xxxx - $yyyy = WOM 16 ; Mark range $xxxx - $yyyy of Intellivision addrs bank-switched. [bankswitch] $xxxx - $yyyy
All other sections are ignored. BIN2ROM does its best to honor the requested memory attributes. If conflicting attributes are specified on a given 2K range, BIN2ROM generally builds a ROM that includes the superset of those attributes. Be careful -- it can lead to some surprises if you're sloppy.
ROM2BIN goes in the opposite order. It takes a ROM, and generates a BIN and corresponding CFG file. The CFG file adheres to the above format, except that it will not output WOM ranges for compatibility reasons. (Neither will the assembler, for that matter.) WOM ranges get converted to RAM, which may or may not be ok depending on your application.