In a long postscript to 65816: Build 4 – Adding Expanded RAM I discussed some alternatives I tried to achieve the following memory map in my ATF22V10C PLD. There’s nothing magical about this memory map, it’s just something I wanted to try, for reasons I’ll discuss in a operating system post if I ever get …
6502: Coding – Just because it works doesn’t mean it’s correct
I ran into an interesting problem the other day trying to upgrade my 6502 build 4 to work with the 65816. I plan to write more on this in another post, but after just one minor change I got build 4 with a 65816 to almost startup my 6502-based Forth operating system. For some reason, …
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6502: My Fourth Build – Testing the New Programmable Logic Device
I completed my fourth 6502 build the other day. It’s a barebones build, mainly intended to test my new PLD-based address decoding/interrupt request design. I didn’t need serial or keyboard support to test the PLD so haven’t included quite a bit of the I/O support that I had in build 3. I’m thinking that I’ll …
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6502: Programmable Logic Devices – Address Decoder Revisited
I mentioned in my last post, Venturing Forth, that I ordered a programmable logic device to better refine my address decoding as my current circuit wastes about 16k on I/O. I worked up the new memory map and PLD files while waiting for the device to arrive. Then, as seems all too common with me, …
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6502: Memory
My first 6502 build, which followed Ben Eater’s 6502 computer project, uses two pretty pedestrian memory chips, an Atmel AT28C256 32k byte EEPROM and a 62256 32k byte SRAM. The EEPROM has a relatively slow 150 nanosecond access time. The SRAM access time is faster at 70 nanoseconds. The significance of these access times become …
6502: Memory Map and Address Decoder
The 6502 microprocessor can address a 64k byte range. As a system designer you must split this address space between RAM, ROM and I/O according to your requirements. My first 6502 build, which followed Ben Eater’s 6502 computer project, used a straight forward memory map with 16k bytes of RAM (address range $0000-$3FFF), 32k bytes …