amberisvibin / chibi-pc09

The pc-09 will be a Motorola 6809 based microcomputer with an advanced MMU, boot scheme, and graphics hardware.

chibi pc-09

Description:

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

Currently there isn't a lot to see here. I am in the process of getting everything from my head onto paper.

Code of Conduct

This project uses the Contributor Covenant. While a code of conduct is not strictly neccesary at this stage, any form of discrimination will not be tolerated. Thank you for understanding.

chibi-pc09's Issues

lowercase not yet properly implemented

issues with keyboard giving uppercase, but needing lowercase when shift not pressed. may be fixed with a tolowercase function

mpl does not fit

this issue will act as a repo for MPL licensing issues

problems:

• MPL doesnt apply well to schematics and open source hardware
• need to determine how to do documentation license
• MPL must be applied on a file-by-file level (less important)

potential solutions:

• separate licenses for different parts
• split into multiple repos

consider changing to Motorola 6809

Benefits:

• proper relative addressing modes
• fully reentrant code
• IRQ stacks entire processor state automatically (Fast IRQ allows customization of exactly what parts of state are stored)
• less cycles per instruction
• position independent code (!!!)
• natively supports 6829 MMU, works almost exactly like proposed TTL MMU (can even do r/rw + x, if it's possible to tell the difference between data read and instruction fetch)
• above features very useful for kernel development

Drawbacks:

• no longer actively manufactured, but is widely available
• max rated freq @ 2mhz (4mhz is possible)(most retro computers didn't get past 2 anyways)
• [MAJOR] no z80 I/O, peripherals will need to be mapped to addr bus (48k RAM? 16 ports of 16 regs, at top of addr?)

NOTE: the motorola documentation is very good

@BOBBYWY comic advertisement

Didn't know how else to contact you.
Appreciate the 6809 project.

For downtime you might like Questionable Content comic on the net. Latest one - https://www.questionablecontent.net/view.php?comic=4527

Based around a coffee shop in MA with a lot of quirky characters, including robots.