Shadow RAM
In the lower 16 KB (# 0000- # 3FFF) of the address space, using a software-accessible register, both ROM and the usually unused lower 16 KB of RAM can be connected. The board is designed to accommodate two sets of 16KB ROM chips, but in practice only one set was used. The register determines where the data will be read from, writing is always done in RAM. Such an organization allows, on the one hand, to organize data loading into the shadow RAM by means of the built-in BASIC interpreter, but, on the other hand, the contents of the shadow RAM can be corrupted by programs that do not take into account its presence. Data in shadow RAM is not lost after a hard reset.
Extended keyboard
The keyboard is a 67-key reed switch. The alphanumeric zone of the keyboard matches a standard AT keyboard with modifier key sets. Alphabetic key layouts are standard (QWERTY and QWERTY). Layout switching is performed with special service keys РУС and Р / А. Additional keys are polled through the keyboard port bits that are not used in the standard model. The total number of software-accessible keys is 56. The rest of the hardware emulate pressing the Caps Shift combinations with number keys and are designed to control the cursor and edit mode.
NMI button
There is a button on the computer case that generates a non-maskable interrupt. This capability, combined with the placement of a debug monitor in shadow RAM, can be useful for program development.
Other circuitry features
There are no waiting loops when accessing RAM, which increases system performance compared to the original ZX Spectrum or its clones such as Leningrad. The RAM regeneration scheme does not depend on the contents of the processor register I, which eliminates the well-known defect of the original model - the destruction of the image when writing values # 40- # 7F to this register.
There is a complete decryption of the input-output ports, along 8 lines of address.
The INT request is automatically canceled by the interrupt confirmation signal, thereby preventing problems associated with the duration of this request.
The number of clock cycles between interrupts is 69888 (like the original ZX Spectrum).
Address signals to the keyboard are routed through the K155LP9 buffers, and not through diodes, as on many other clones. This eliminates possible interference to the system bus from the keyboard.
The signals of the video controller are generated under the control of ROM (microcircuits K155RE3 and K556RT4). Synchronization exactly matches the original ZX Spectrum.
The computer is powered by 24V direct current (power supply is included in the delivery set). A pulse voltage regulator is located in the computer case, which generates the main supply voltage + 5V and additional + 12V. The computer is not sensitive to electrical noise such as turning on the refrigerator. The load capacity of the stabilizer along the + 12V circuit is very small, but sufficient to power the KR1818VG93 disk drive controller microcircuit.
Compatibility
The computer is partially compatible with the original. The main source of problems is the changed contents of the upper ROM addresses, which causes malfunctioning programs (for example, the Bomb Jack game) that use the ROM as the source of the address of the IM2 interrupt handler.
Basic software
In addition to the built-in BASIC interpreter, the computer is supplied with a tape cassette with tst, MZ80 and RECLAMA programs.
Built-in BASIC
The ROM firmware contains a variant of the original ZX Spectrum ROM, adapted to the advanced capabilities of the BC. When loading, the message "Basic system ver. 2.0" is displayed.
Characteristics:
Russification: 7-bit KOI-7 encoding is used with code page switching by outputting control codes. Modified ROM routines for entering characters from the keyboard and displaying them on the screen. The tables of symbols of the Russian character generator are located in the upper ROM addresses that are not involved in the original.
Implemented the traditional for ZX Spectrum input of BASIC language service words by pressing one key, but the arrangement of words on the keyboard differs from the original.
NMI handler: instead of the standard handler, a custom handler has been implemented that activates shadow RAM, which causes an implicit transfer of control to the code located in this RAM.
Known bugs in the original ROM have not been fixed. At the same time, the differences in the BC architecture required the correction of some other problems that did not manifest themselves on the original architecture, but could cause partial data corruption in the shadow RAM.
Monitor MZ80
The small operating system MZ80 is designed to execute and debug programs in machine codes and implements the following functions: control of the input-output system, loading programs, launching programs, generating and copying files (programs), debugging, and service capabilities.
The monitor is located in the shadow RAM and is activated by the user using the NMI button.
The capabilities of the MZ80 as a debugger are not very diverse. For example, it lacks features such as disassembling the contents of memory or stepping through programs.
Incorrect work with the contents of the processor R register and the peculiarities of returning to an interrupted program do not guarantee recovery after a non-maskable interrupt.
Tst program
The program is designed to test the functioning of the main units of the BC: keyboard, color and brightness control signal generator, sound generator, border color control signal generator, RAM and ROM, input-output devices, cassette tape recorder signals.
RECLAMA program
Demo on Laser BASIC.