Z80 Project Memory Expansion
Last updated: 14 Mar 2011 - 00:11
The Mark 2 Z80 project featured a pair of memory slots each capable of taking up to 32K of RAM. This meant that a full 64K of RAM could be used because the ROM image is copied from the PIC's memory at boot time. However that didn't seem like much to me so a banking system and MMU were in order. (Also I found I couldn't get the 32K RAMTRON FM1808 chips in DIP any more!)
The new system is designed around 512K SRAM chips, up to 8 of them, although I've only wired up two sockets for 1MB of RAM for now. If you know your base 2 maths you should realise that I need a 22 bit address bus for the 4MB address space. This initially seems like an odd size but the design is based on the internals of the Amstrad NC100, a portable Z80 machine from the early '90s. The top 8 bits of the 22 are provided by one of four latches. Which of these latches provides the last 8 bits is decided by the Z80 address lines A14 and A15.
This means the physical memory space of the Z80 is made up by four 16K
Z80 Project Updates and Revision Control
Last updated: 26 Mar 2010 - 22:39
It's been a (very) long time since the last update to this project. I've been busy on loads of new stuff for the project. The system is now capable of booting a binary from SD card or USB memory stick formatted in either FAT16 or FAT32. There's also full PS/2 keyboard support, a much more sophisticated interrupt control mechanism and a VGA display now.
I'm planning to go through all these new features and write-up how I've got there but for now I'm just going to do a summary. Since my last update, I have built a basic chassis to contain the growing number of boards and hold all the connectors, there's a new connector board which has the USB host sockets, an SD card slot, PS/2 keyboard connector a composite video output connector and a 3.3V and 5V power supply. This connects up to the original pair of boards providing power and two LEDs (one is driven by the PIC as a "hard disk light" when accessing the SD, the other is driven by a transistor from the Z80's HALT line), the 5-pin