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updated readme to current plans

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2024-10-22 10:32:05 -04:00
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# chibi pc-09 # chibi pc-09
![GitHub last commit](https://img.shields.io/github/last-commit/amberisvibin/chibi-pc80) ![GitHub last commit](https://img.shields.io/github/last-commit/amberisvibin/chibi-pc80)
![OS9 Mockup](sprite/320x200_os9_mockup-logo_rainbow_2x.png) ## Description
## This document is no longer accurate and will be updated in the future. The PC-09 will be a 6309 based microcomputer with a 74LS612 MMU, uPD72020 graphics and keyboard input.
The MMU will allow up to 2 megabytes of I/O to be paged into the address space. Pages are 4k. System storage will be paged into the address space as well, as it will be either EEPROM or flash.
The uPD72020 is a very advanced graphics chip for it's time, capable of accelerated drawing of lines, shapes, fills, and characters. It can be coerced into outputting a VGA signal at 640x480 and *maybe* 800x600.
## Description: Prototype 1 is currently in progress. It will be a much simpler system. It will have no MMU, just the CPU, some RAM, some ROM, and a serial port.
The pc-09 will be an extremely expandable Motorola 6809 based microcomputer that can run OS-9 of off an SD card:
- On boot, the coldboot ROM will search an SD card for a bootfile, load it into RAM, and execute it. The bootfile will be a BIN file that is copied directly to RAM. This bootfile can be copied and moved the same as any other file, allowing for easy backup and distribution of boot code.
- It will come with 64k of RAM by default. These are split into 2k RAM blocks. The first page is the Kernal Root Block. The second-to-last page is the I/O Block. The last page is the Vector Block. All other blocks are normal RAM. A maximum of 58k of address space is program-accessible, without an MMU.
- The 6829 MMU creates 4 different address spaces, with the kernal occupying address space 0. With the MMU, only the kernal process will need the I/O and Kernal Root Blocks. Multiple MMUs can be combined to create up to 32 address spaces. The MMU has a 10-bit output, but the maximum amount of addressable RAM will be 1mb, as the upper address line is used for marking r/rw, as outlined in the 6829 datasheet.
- The I/O Block is sub-divided into 128 devices, with 16 addresses each. Address 0 will be a descriptor, so that the coldboot ROM can address it with a defined protocol.
- The graphics system will be more advanced than most homebrew computer projects. It will have a custom TTL logic graphics card that will be able to display 320x200 in 256 colours over VGA. There will be 64k of double-buffered VRAM, interfaced via commands. The buffer will not copy unless a 'done' command is sent. This way, if the frame is not ready, the previous frame will be displayed, preventing tearing. These commands will include an auto-increment function, allowing entire bitmaps to be copied with ease. It will also have a text mode, utilizing bitmap copy from the character ROM. [SUBJECT TO CHANGE]
- It will run OS-9, with pc-09 specific drivers and memory management. This should be invisible to OS-9 programs that use only system calls.
## Progress ## Progress
Currently there isn't a lot to see here. I am in the process of getting everything from my head onto paper. I had started wiring together a board for prototyping, but it was destroyed in hurricane Helene. I am now recreating it in schematic form in order to order a PCB.