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Last modified on 3 August 2019, at 06:47

Z-Machine

Revision as of 06:47, 3 August 2019 by Arnoldemu (Talk | contribs) (→‎Technical information)

The Z-Machine is a text based interpreter which runs text adventures made by Infocom and Z-code files.

CPCZVM is a Z-Machine interpreter for the CPC. It is a modification of ZXZVM, an interpreter made by John Elliott for the PCW. The Amstrad version was converted by Kevin Thacker. There are two versions.

  • A version for CPM
  • A version for AMSDOS

CPM version

CPCZVM for CPM features a virtual memory system supporting both the Z-machines RAM and ROM. Technically it will run under CPM2.2 but currently doesn't. It runs under CPM+.

Running an adventure

  • Copy CPCZVM.COM, ZXZVM.BIN and the game file (e.g. ADVENT.Z3) to media which has at least 64KB free. All of these should be in the same location. It is advised to use CPM+. CPM2.2 is supported but there is not enough memory. CPCZVM may not be compatible with other CPMs.
  • Boot into CP/M.
  • Select the disc (e.g. |B)
  • Type: CPCZVM ADVENT.Z3 to start.

NOTE:

  • Transcript is not currently supported.
  • Italics, Bold and other font styles are not currently supported.
  • Using a RAMdisk under CPM+ doesn't make much of a difference to the speed of loading.

Technical Information

  • The program reads the first 64KB of the game file, padding with 0's if less than 64KB, and writes out a SWAP.DAT file. A Z-machine has a maximum of 64KB of RAM. Therefore the disc which CPCZVM is run from must have at least 64KB free and be writeable. When the RAM is written to this swap file is updated. This swap file is also re-generated from the game file when the game is restarted.
  • Part of the CPC's memory is used for the virtual memory cache. A page table describes which virtual memory blocks are loaded and their physical location within RAM. This is initially populated from the start from the game file. When a read is done, the virtual memory address is looked up in the page table, and if it is mapped to physical memory then the physical memory is read. A write is similar to a read with an additional flag being set if a write has been made to a mapped page. If a virtual memory page is not currently loaded, then depending on the metric used, a page is chosen to be evicted. If the page has been modified it is committed to the swap file. Then the new page is loaded into the same physical location. During game play, any access to virtual memory in the first 64KB comes from the swap file, and any accesses above 64KB come from the game file. In this way the virtual memory supports both Z-Machine RAM and Z-Machine ROM.
  • CP/M is used because it has a random access filesystem compared to AMSDOS which doesn't. The random access is required to read blocks from the swap and game file when needed.
  • Some versions of the z-machine require that the screen can be split into more than 1 window. The VT52 terminal emulation in CPM+ on the CPC is unsuitable because it only supports a single window compared to the VT52 terminal emulation on the ZX Spectrum Plus 3 and the PCW. Therefore it was decided to use the firmware TXT functions to provide this. I am using USERF to access these.
  • The code does support Amstrad's CPM2.2 using "ENTER FIRMWARE" to call the firmware functions however the TPA is not large enough to support running the games.
  • Because of my use of userf and ENTER FIRMWARE this means CPCZVM may not compatible with other CPM (e.g. Vortex, Dobbertin) at this time. I have not tested to confirm.
  • Disc accesses use normal CP/M functions so it should work with other devices such as RAM discs, Harddisks etc.

AMSDOS version

CPCZVM for AMSDOS requires additional Dk'tronics compatible memory (e.g. X-MEM ). The entire game file is loaded into available RAM so it is recommended to use a 512KB RAM expansion.

Running an adventure

  • Copy CPCZVM.BAS and the game file (e.g. ADVENT.Z3) to media. Both of these should be in the same location.
  • Make a simple BASIC loader with this form:
10 a$="advent.z3"
20 chain"cpczvm"

and save it: 

save"advent
  • Now you can run your adventure:
run"advent

NOTES: 

- advent.z3 is the name of the game file. It should not have a header. 
- a$ must be used
- advent.bas is the name of your basic loader
- It is advised to use a large ram expansion (>64KB)

Technical information

  • The CPCZVM.BAS contains a fixed basic stub followed by the machine code. The BASIC stub must not be modified or it will break CPCZVM.BAS. Using a BASIC stub means I don't need to re-enable any ROMs (like I would if RUN was used) and the current drive and filesystem remains active allowing hard disks and other DOS to be used. In addition I can pass HIMEM to know if there is enough RAM free (expansion ROMs can modify HIMEM to allocate work space) and a string variable containing the game filename.
  • The program uses the firmware except TXT WRITE CHAR is patched so that bold, italics and inverse is supported. The font is manipulated as it is drawn.
  • Transcript goes to the printer.
  • The program detects the amount of additional RAM and will warn if there is not enough when loading the game file.
  • The program only supports Dk'Tronics compatible RAM.
  • The entire game file is loaded into extra ram using "CAS IN CHAR" functions. This is used because the extra memory is paged in 16KB at a time and CAS IN DIRECT reads in an entire block and the game files exceed the size of 16KB and do not require an AMSDOS header to be added.
  • The BASIC boot program allows a game filename to be passed to CPCZVM.BAS without requiring it to be modified. The use of CHAIN keeps the defined variables and HIMEM.
  • This version should run from X-MASS or other hardware.

Downloads