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RB6588






What is RB6588?

RB6588 is a piece of hardware based on the Godil, meant to replace the original 8088 of an IBM-XT (compatible).

For the rest it is almost the same as RB6502. The only two differences is the way the hardware is soldered and the core of course.


The core

The first problem we bump into is the number of address lines: the 8088 has 20 address lines, the 6502 only 16. The idea is to evolve RB65xx into a CPU with more registers, opcodes and address lines anyway but to be able the basic core right now we need another solution. And this solution is adding a very simple MMU.

One thing a MMU, Memory Management Unit, can do is that it enables you to handle more memory then the original CPU can. Read the page about the 74LS612 MMU first to find out what a MMU is and does.

In this case I want to start with very simple, non-programmable MMU. It does nothing more then translating some 6502 ranges to 8088 ranges:

6502 8088 Function
$E000 / $FFFF $FE000 / $FFFFF MDA Video RAM
$CC00 / $CFFF $000 / $3FF I/O
$0000 / $CBFF $00000 / $0CBFF RAM

The next step is to expand the RB65 core with extra address lines. Another idea is to increase the size of the registers. 16-bit registers enables to replace the 80286 of an IBM-AT (compatible) with the Godil :)





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