PCjs Machines

Home of the original IBM PC emulator for browsers.


Juku E5104 Test Machine

[PCjs Machine "juku"]

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Selected Information

From the “Juku E5105 Microcomputer User Manual (1988)”, p.10:

Juku’s memory consists of 16K (expandable up to 64K bytes) of non-volatile memory and 64K bytes of random access memory. 10K bytes of random access memory make up the so-called video memory, where a video image is formed on the TV screen based on the stored information.

Since the used microprocessor (KP580MK80) allows direct addressing of the memory only to the extent of 64K bytes, 4 different memory addressing methods have been introduced for the sake of efficient use of the address space and further expansion of the memory. The necessary mode is selected with the two lowest bits of the system gate (address 02H), according to the following table:

Mode 0

Applies when the computer is turned on immediately after the RESET signal. Within the first 16K bytes of the address space, variable and non-volatile memory are located in parallel. With the read command, the data is read from the permanent memory; with the write command, the data is stored in non-volatile memory. The last 10K bytes of random access memory (or rather 640 bytes less) includes video memory.

Mode 1

In this way, other memory starts at address 0000. In the last 10K bytes of the address space, the memory depends on the type of access operation. With the read from memory command, the data is read from the permanent memory, but with the write command, the take data to video memory. You can’t do it this way turn towards the first 6K bytes of the non-volatile memory block.

Mode 2

Intended to refresh memory. The memory zone 4000-BFFF is formed by the so-called memory window. When accessing these addresses, the internal memory of the computer is not selected, the access is directed to an apparent 32K byte memory page. There can be more than ks of such external memory pages by thumb, but in that case they must be equipped with appropriate hardware means that allow them to be selectively opened and closed. The non-volatile memory is located at the end of the address frame, parallel to the video memory. The turning modes are the same as in mode 1.

Mode 3

When using mode 3, non-volatile memory is disabled. It is possible to write to and read from the video memory.


Up to 64K bytes of non-volatile memory can be achieved with 8K bytes of non-volatile memory when using chips (K573РФ4,K573РФ6). In the future we will look at the so-called computer. puff variant - s. o. 16K permanent memory and 64K RAM. In the case of a larger volume of permanent memory, the border between random access memory shown in mode 0 shifts «downwards». The rest of the memory allocation blocks will remain the same.