"MTX Plus⁺" Speculator⁺

Introduction

Tony Brewer developed a product called Speculator for the original MTX computer, it allowed an MTX512 or RS128 to emulate a Sinclair ZX Spectrum sufficiently to run a limited number of Spectrum games on the MTX. A version of Speculator was also released for the Tatung Einstein which supported a larger number of games, loaded from disk.

To use the MTX version, the user first loaded the Speculator tape which contained the Spectrum emulation code and presented a menu of  20 games that the user could load from the original Spectrum tape and play on the MTX. (My Speculator page describes Speculator in more detail.)

With assistance from Tony, Andy Key has added Speculator support to MEMU, his excellent Memotech MTX emulator.

Tony has now helped us to integrate Speculator with MTXPlus+. The Memotech version of Speculator used a couple of 74 series TTL chips, a couple of PLDs and a 2K SRAM, which were used to emulate the Spectrum keyboard.

With the extra resources that MTXPlus+ has available, the MTXPlus+ version does not require additional hardware and has a number of enhancements over the Spectrum version :-

• Faster operation
  • Optimised for running at 8Mhz
  • Uses fast paging of MTXPlus+ extended RAM
• Compact Flash media support
• Uses a copy of the actual Spectrum ROM

With it's significant improvements over the original, and in line with the MTXPlus+ system name, Spectrum support in MTXPlus+ has been christened Speculator⁺.

MTX Speculator

In the ZX Spectrum, the Z80 CPU was clocked at 3.5MHz, in the MTX computer, the clock speed was 4MHz, although only slightly faster, the design of Speculator allowed emulated Spectrum games to run at an acceptable speed.

Running on the MTX, Speculator had to work its magic in the 64k of RAM available in an MTX512. The Speculator code moved 16kb of banked RAM from its usual position at Page 1 (8000h to BFFFh) of the MTX memory map to Page 0 (0000h to 3FFFh) in RELCPMH=1 mode, i.e., paging out the MTX ROMs and making a full 64kb of contiguous RAM available.

Most Spectrum games bypassed the Sinclair ROM and used very few of the native ROM routines, so Speculator did not need to emulate most of the Sinclair ROM. A very few functions, such as the Clear Screen (CLS) function were needed, and since Speculator could not use any copyrighted Sinclair code, the first 16kb of RAM had to have an equivalent function located at the same address as the original so that the game could find the code in the emulated ROM. 

ZX Spectrum Memory Map

 

Address Range	Contents
0000h - 3FFFh	ZX Spectrum ROM memory range
3D00h - 3FFFh	Spectrum character shape table
4000h - 57FFh*	Spectrum display file
5800h - 5AFFh*	Spectrum colour attributes
5B00h - 5BFFh	(printer buffer)
5C00h - 5CBFh	Spectrum System Variables
5CC0h - 5CCAh	(reserved)
5CCBh - FFFFh	Free for Spectrum game code
2000h -  5CFFh	Speculator Supervisory code
(For more details of the Spectrum ROM, see my ZX Spectrum page)

* The Spectrum's screen characters and colour attributes are stored in RAM and a major element of the Speculator code was to move the screen data between the system RAM and the Video RAM used by the MTX's TMS 99xx VDP through the VDP I/O ports.

Whilst the Spectrum and MTX video resolution's were the same, at 256x192 pixels, the MTX VDP had better colour capabilities and required more information to be stored in VRAM than in the Spectrum's memory mapped display area. Speculator had to move 1B00h bytes into the 4000h bytes of MTX VRAM, ideally, this would have been done every time that the Spectrum screen was updated, but the overhead of doing this would have been too great to allow acceptable game play.

Instead, Speculator used some clever tricks to optimise the screen updates using interrupts. The Z80 interrupt on the Spectrum is triggered at the start of the screen refresh vertical blanking period which takes place at 50Hz, it was possible to update 1/6th of the screen in each interrupt cycle. Depending on where more screen activity took place in a given game, Speculator could update some areas of the screen more often than others. For example, most of the action in Starion takes place in the top two thirds of the display, so Speculator refreshes the bottom third at a lower frequency, allowing the rest of the screen to run close to the speed of the original.

Colour updates were optimised too, the 8 Spectrum colours were mapped to the closest colours in the TMS VDP palette of 16, but only those attributes that had changed were updated in the VDP's VRAM.

MTXPlus⁺ Speculator⁺

When Speculator was released, it was not possible to legally copy the Spectrum ROM or to include portions of it in Speculator, however, Sinclair Research sold the rights to their computer products, including the Spectrum, to Amstrad in 1995. Amstrad still owns the copyright for the Spectrum ROMs, but since 1999, has allowed free copying and distribution of the ROMs. For details, see this record of a post on the comp.sys.sinclair forum archived on the World of Spectrum website. This means that Speculator⁺ can legally use a copy of the original ROM and doesn't need to have code to mimic the original Spectrum code.

As with the original Speculator, the system runs with the normal MTX ROMs paged out and the full 64k Z80 address space taken up with RAM, with the lower 16k of RAM being occupied by Spectrum ROM code (except during NMI periods).

Like the original, Speculator⁺ needs to convert Spectrum hardware specific data in RAM, such as screen data and attributes, into their MTX equivalent and it does this in a similar way to the original, using the Z80 NMI function. In Speculator⁺, this is done by paging out the Spectrum ROM and paging in the Speculator⁺ specific code during the NMI period.

Since Speculator⁺ now includes a copy of the Spectrum ROM in RAM space, an additional measure is needed to guard against one of the "tricks" used by Spectrum programmers corrupting the "ROM" data. As the Spectrum memory map (above) shows, the display data file is immediately after the 16k ROM. Some Spectrum code writes to addresses below 4000H when updating the top-left of the display, safe in the knowledge that the ROM cannot be altered. In other words the programmers didn't bother with rigorous boundary checking when it didn't really matter. To prevent data writes to the ROM when it is in MTXPlus+ RAM, Speculator⁺ write protects the lower 16k of RAM, except during the NMI routine. This is done by putting the RAM WR line under the control of the Speculator+ CPLD, rather than the CPU, and conditioning the write signal based on the target memory address when Speculator⁺ is running.

Memory Management

Speculator⁺ uses the MTX/MTXPlus⁺ memory paging system to control its operation, a quick overview of the MTX Page Port will help understanding of how this works. More details of MTXPlus+ memory paging can be found on this page.

Output port 0 is the MTX Page Port, this 8 bit port is used to switch the system between ROM enabled mode and RAM only mode and to select the ROM or RAM page used for memory access.

Without paging, Z80 Address bits A₀ to A₁₅ can address up to 2¹⁵ bytes i.e., 64kb of memory. Similar to the original MTX, MTXPlus+ supports up to 512kb of RAM using paging, up to 16 pages, each of size 32kb, can be paged into the Z80's address space. Address bits A₀ to A₁₃ can address up to 2¹³, i.e., 16k memory locations; A₁₄ and A₁₅ are used to point to the location of the 16kb block in the memory map. When A₁₄ and A₁₅ are set, the common 16kb block is selected, selection of other pages in the RAM is done using A₁₄, A₁₅ and the Page Port RAM bits to map the extended RAM into the 16 bit address space.

Using the expanded memory of MTXPlus+, Speculator+ has access to a protected block of 64kb of RAM that was not available to the original Speculator. This block of RAM can be accessed in both ROM and RAM modes (RELCPMH = 0 or 1) using appropriate page port values. 

When MTXPlus+ starts, it does so in ROM mode (RELCPMH=0) and 4x16kb blocks of high memory are reserved for Speculator+ use by placing them above the empty block in Page 13 (D) of the memory map.  When MTXPlus+ is running under control of the ROMs, these hidden pages are not used and just reduce the apparent total RAM to 448kb.

A 512kb RAM uses 18 address lines to access the 2¹⁸ memory addresses, the MTXPlus+ address decoding logic sets the values of the address lines to target the appropriate block in the 512kb RAM. The following table, extracted from the memory decode logic, shows the paged RAM blocks used by Speculator+. 

When Speculator+ is running, the contents of these reserved locations is as shown in the table. Block number 31 is the common block of RAM, from 48k to 64k, visible in all memory modes. When Speculator+ is running, the upper 16kb of Spectrum RAM (32-48k) is mapped into this space.	Block  Contents  27  Spectrum RAM (0-16k)  28  Spectrum ROM    29  Speculator+ Code    30  Spectrum RAM (16-32k)   31  Spectrum RAM (32-48k)
When the system is in ROM mode (RELCPMH=0) and the value "0E" is written to the Page Port, pages 28 and 29 are both visible, allowing the Spectrum "ROM" and Speculator+ NMI "ROM" to be loaded from CF.
When the system is switched to RAM only mode (RELCPMH=1), the memory memory map is changed to duplicate two of the Speculator RAM blocks, making them appear twice in the address space, effectively meaning that the 48k of Spectrum RAM is common when the Page Port is switched between "8E" (Spectrum "ROM") and "8F" (Speculator+ "ROM").
The NMI "ROM" is paged in and out to refresh the Spectrum display, the same block of RAM used to replicate the Spectrum 48k RAM is visible to both the Spectrum "ROM" and the NMI "ROM".  When the Spectrum "ROM" is switched in, the lower 16k of RAM is write protected to simulate the effect of an actual ROM being present in this address range.

When the Z80 NMI pin goes low, the CPLD has the same effect as writing "8F" to the Page Port, i.e., block 29 (containing the Speculator⁺ "ROM") replaces block 28 (the Spectrum ROM) in the lower 16k of RAM and the write protection is removed from this address range.

When the Z80 NMI pin goes low, the CPU will execute the instruction at address 0066h, which will be the Speculator⁺ NMI code to update the Spectrum screen, i,e., the MTX VRAM is updated from the Spectrum screen data starting at address 4000h in the 16-32k block of RAM in block 27.

 I/O Ports

Speculator and Speculator⁺ need to emulate a few Spectrum I/O ports to be able to interface with the Spectrum program, these are summarised in the table below :

XX = Original Spectrum 48k port

XX = Speculator⁺ port