Monday, June 23, 2014

6502 Virtual Trainer: Update

Lately I have been working on several different projects and I have made a lot of progress on the 6502 Virtual Trainer. The first step was connecting the PC interface to an MSP430 over UART. This was easy because I have done it in other projects. Microchip let me sample some MCP23S17 IO expanders and I was really excited to be able to try them out to read and write the pins of the 6502. Unfortunately, they were a little finicky to get going because the datasheet incorrectly lists the reset pin as an output instead of an input.  They do save a lot of space, though, because five shift registers would be needed to replace them. After that was solved, I was able to single-step the processor as I intended but only at about 90 cycles per second. For comparison's sake, with the processor doing nothing else, that is a little too slow to type comfortably. At the beginning I knew this would be slow but that is much slower than I expected. The bottleneck seems to be the USB<->serial layer since the program sends lots of small packets of data instead of small numbers of large packets. To speed this up, I send 10 bytes of op codes every packet. Then the MSP430 can use the data sent if it needs to read a byte that is near. It will have to request more data if a jump takes place or data is read from another location in memory but in general this improvement led to a speed up of about 150 cycles per second. This was still not as fast as I had hoped for so I decided to keep a copy of all of the memory on an SPI SRAM chip. For this I used a 23LC1024, also from Microchip, which holds 128k. Using an SPI SRAM might seem to make less sense than just hooking up a typical parallel SRAM but by letting the microcontroller control the memory I can hold extra information about the data such as which bytes have not been initialized and which should be ROM. This will let me break when reads and writes that would be illegal are performed. Even during testing this turned out to be useful as it is easy for a beginner to assembly to write something like LDX $FF instead of LDX #$FF. As a compromise the MSP430 runs the 6502 as fast as possible then pauses and updates the PC every 200ms or when 64 bytes of memory have been modified, whichever comes first. Another way to speed things up was to increase the COM port speed from the 230,400 bps max of the MSComm control I'm using with Visual Basic 6 to 1,000,000 bps. At 1,200,000 bps the MSP430 could not keep up running the 6502 and reading data. With these changes I can run the 6502 at about 12,000 cycles per second. This is still pretty slow but fast enough to play with for learning purposes. To test things I made a few programs including one that draws a faded red pattern like below:

To assemble the programs I am using CA65 which is part of CC65. Instead of compiling to binary I only generate a listing file. From that I can read not only the generated source but also the names of labels and what lines of the source the code was generated from, so that I show all of this information as part of the memory viewer.

Another part I worked on is a window to load code and set the color of memory in the memory viewer in the main window. Regions of memory can be labeled and colored, and double-clicking on a title jumps to that location in memory. Regions can also be marked as code or read-only so the program can break on incorrect accesses.

Next I plan to improve the cycle diagram at the top of the main screen to show which op-codes and which addresses were being accessed, as well as the status of auxiliary pins. Someday I would like to make another calculator using the 6502 and this will help me learn how it works. Unfortunately, I was not able to solder everything to perfboard before I left Kyrgyzstan. Now that I am back in the US it marks the end of the time I started this blog for. I look forward to doing more projects now that I have access to more components and better equipment.

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