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The biggest limitation of any 8 bit CPU such as our beloved 65C02 is the amount of memory that the CPU can address. With 16 address lines, the addressable memory is maxed out at 64k. All ROM and RAM has to be crammed into there. With the 6502 being a memory mapped architecture, IO devices need their addresses there, too.

In order to expand the amount of usable memory, some trickery is necessary. For example, the developers of the C64 came up with a rather clever hack to cram 20k of ROM and full 64k of RAM and IO area into 64k address space by introducing a register that enables the programmer to switch off the ROM, giving access to the underlying RAM. When the ROM is enabled, writes to the addresses go into the RAM below. We decided to mimic this behaviour in our current implementation of the Steckschwein glue logic.

Since our implementation of FAT32 now supports reading a file byte for byte, a little rework of the file handling in our version of EhBasic is in order.

In the past, we only could read or write a file as a whole, relative to the location in memory where the according pointer pointed to. We used this in EhBasic to save and load BASIC programs by dumping and reloading it’s binary representation from memory. While this works well, this approach has the major disadvantage that the saved program will be incompatible with other versions of EhBasic or even with our own when the token list is changed, which happens when adding new commands.

The way the PS/2 keyboard is handled has always been something we were never quite happy with. The key points being:

• The PS/2 controller had no way of signalling that there has been a new keystroke, the buffer had to be polled via SPI.
• The PS/2 controller had no way of talking to the keyboard and had to rely for the keyboard to initialize itself properly. Also, typematic rate and delay could not be set, as couldn’t the states of the keyboard LEDs.

Although mid- to long term, we likely might “upgrade” to USB anyway, but not without having done PS/2 right first. So, I will talk about integrating IRQ handling, and in a follow up post Marko will talk about how he got the PS/2 controller talking to the keyboard.

Recently, Michael Steil published a blog post about connecting NES and SNES Controller to a 6502-based system showing how to use NES and SNES controllers on a C64 without the need for any special hardware, by just connecting them to the C64’s user port.

Why not use his approach and adapt it to the Steckschwein? The Steckschwein has a User Port, too, albeit a very different one as the C64. Basically, the Steckschwein-User-Port consists of the complete Port A of the VIA, plus the /RESET and /IRQ lines. Also of course, VCC and GND.

Chuck Peddle, the main designer of the 6502, has passed away on Dec. 15th, 2019.

Peddle was one of the engineers that developed the 6800 at Motorola. He later went to MOS in order to implement his vision of an 8bit CPU for way less than $300, which was Motorola’s price for the 6800.

This idea of a cheap but powerful CPU materialized as the 6501, and finally the 6502. That very chip, which started the microcomputer revolution, and on which both Marko and myself began to write our first code ever at an early age. BASIC at first, followed by assembly language later.

Back from the VCFB (Vintage Computer Festival Berlin) 2019 where we had good talks, met interesting people and got new ideas. Especially from Michael Steil who just asked the simple question “How you can develop software for the Steckschwein without an emulator?”

With the answer in my mind I felt a little annoyed and also “triggered” at the same time… However, Michael Steil was nice enough to strip down his Commander X16-Emulator into a barebone 65c02 computer emulator, so all we had to do was to implement our memory map (easy) and borrow a V9958 video chip implementation from blueMSX and implement it into the emulator (hard).