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PREHER-816

After dropping the first Heritage/1 effort, I came with an 8-bits candidate called PREHER/816. That approach didn't last for long.

64KB of memory should be enough for experimenting with low-level software, even with peripherals mapped in that space. However, we can extend the range by using a Memory Banks scheme.

We can store the Bank Number in a 8-bits hidden register. An instruction maybe termed SETBNK (number) would be employed to switch banks. This allows for 256 banks, that is 16MB of memory.

This could be also exploited as a rudimentary support for protected multi-tasking. If the OS manages to allocate different processes in different banks (64K per process) and development tools (Assemblers, Compilers and Linkers) are designed not to allow SETBNK instruction in "Application Development Mode", then it would be nearly impossible for application code to trespass process boundaries.

As per address lines, the internal address bus (A-BUS) would still 16 bits whereas the external address bus (that in U-BUS) would have 24 lines, the 8 most-significant ones being fed from the BANK register.

I am considering this idea since Heritage/1. If all registers are built using presettable up/down three-state binary counters, then I can execute INC/DEC instructions in only one clock cycle (T1). This might represent a notable improvement of the overal execution speed since a typical program employs a good deal of time looping through counter variables.

The only problem is cost. Until now, I were thinking of building registers with 74HC374 chips (one chip per 8-bits register, two for 16-bits registers). With presettable counters such as the 74LS169 (and 74HC244 as buffers because this counter doesn't provide three-state outputs) the cost per register may increase by a factor of three.

An alternative is to provide INC/DEC capabilities to only a few registers (i.e. PC, SP and C) leaving the rest not incrementable... or we can increment those others via ALU, in which case we tell programmers (which programmer apart of myself?) to use the register C as a loop counter becouse it is faster.