The CPU needs to first write a magic sequence of bytes in order to unlock the ROM. The ROM chip has a read/write input pin just like an SRAM, but writing to the Flash ROM is more complicated. One feature I could have added to the monitor program, but didn’t, was the ability to reprogram the Flash ROM. I was even able to get Lee Davison’s ehBASIC for 68000 working, which provided a few hours of fun. Using the monitor program, I was able to load other programs in binary or Motorola S-record format over the FT245 link, store them in RAM, and execute them. Wichit Sirichote’s zBug monitor provided a good starting point for my own ROM-based monitor/bootloader. Once the hardware build was done, I began writing some simple test programs in 68K assembly language. Everything was built incrementally, one wire at a time, while staring at chip datasheets. #BREADBOARD SIMULATOR EMULATOR MAC SERIES#The rest of the protoboard is filled with assorted 7400 series parts and one PAL, which are used for address decoding, interrupt arbitration, and other basic glue logic. The protoboard has eight LEDs for debugging, which are driven from a memory-mapped 74LS377 register. The initial version of the hardware had no timer interrupt, but I later discovered it was essential in order to get Linux working correctly. When there’s an incoming byte waiting in the buffer, it triggers a CPU interrupt.Ī 555 timer provides the only other interrupt source, generating a regular series of CPU interrupts at roughly 100 Hz. The FT245 has an internal 256-byte buffer, which helps smooth out the communication. Reading from its address fetches the next incoming byte from the PC, and writing to the address sends a byte out to the PC. On the 68 Katy side, the FT245 appears as a byte-wide I/O register mapped into the CPU’s address space. Windows Hyperterm or another similar terminal program can be used to communicate with it, like an old VT100 terminal. #BREADBOARD SIMULATOR EMULATOR MAC SERIAL#On the external PC, it appears as a virtual serial port. That’s pretty slow, even in comparison to 68000 systems from the early 1980’s, which were typically 8 MHz or faster.Īn FT245 USB-to-FIFO module provides a communication link to another computer. My 68008 runs at 2 MHz (it was unstable when tested at 4 MHz), providing similar performance to a 0. Neither chip has any on-board cache, so half the memory bandwidth leads to roughly half the performance. The 68008 requires fewer memory chips thanks to its 8-bit data bus, but the trade-off is that memory bandwidth is only half that of the 68000. The standard 68000 CPU has a 16-bit data bus, so it normally requires at least two 8-bit RAM chips and two 8-bit ROM chips. It’s paired with a 512K 8-bit SRAM, and a 512K Flash ROM (of which 480K is addressable – the remaining 32K is memory-mapped I/O devices). The CPU has 20 address pins, allowing for 1 MB of total address space. The heart of the system is a Motorola 68008 CPU, a low-cost variant of the more common 68000, with fewer address pins and an 8-bit data bus. It took about a week to assemble and wire up all the parts on a solderless breadboard. Here’s a look at the final version of the hardware. I’ve named this plucky little machine 68 Katy. Having a full-fledged multitasking OS running on this ugly pile of chips and wires is a thrill, and opens up all kinds of interesting new possibilities. #BREADBOARD SIMULATOR EMULATOR MAC SOFTWARE#After banging my head against the wall with problems ranging from the inductance of pushbutton switches to memory leaks in the C standard library, it finally works! I’ve built several other DIY computer systems before, but never took their software beyond simple assembly language programs. What does it take to build a little 68000-based protoboard computer, and get it running Linux? In my case, about three weeks of spare time, plenty of coffee, and a strong dose of stubborness. Steve on Floppy Emu Disk Emulator for Apple II, Macintosh, and Lisa.Race Fan on Yellowstone Universal Disk Controller for Apple II. #BREADBOARD SIMULATOR EMULATOR MAC PRO#Chris on Saleae Pro 8 Logic Analyzer Review.Yellowstone Universal Disk Controller for Apple II Floppy Emu Disk Emulator for vintage Apple II, Macintosh, and Lisa
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