- APPLE2 6502 EMULATOR TEXT BASED GENERATOR
- APPLE2 6502 EMULATOR TEXT BASED CODE
- APPLE2 6502 EMULATOR TEXT BASED PLUS
On the later 80-column motherboards, they used an even smarter approach where the video hardware sees the memory as 2048 words of 16 bits and decodes the characters at twice the speed.
APPLE2 6502 EMULATOR TEXT BASED GENERATOR
Commodore figured out that with 40 characters per row, and a video line time of about 64 microseconds, it would be possible to run the CPU at about 1MHz and design some circuitry that alternates the access to the video hardware between the CPU and the video generator hardware twice per clock cycle. Just like in many other early 6502 computers, the entire system was built around the video hardware. In the original machines, video was the most complicated part of the hardware, because the RAM needs to be accessible by the 6502 while it's generating a constant signal. One cog is in control of enabling the SRAM based on the address bus of the 6502, and because I already had that module, it was a matter of copying it and adding lines to initialize the module and start it up.
APPLE2 6502 EMULATOR TEXT BASED PLUS
Though it would be possible, in theory, to use hub RAM as 6502 RAM for the PET/CBM project, it's much easier to just use an external SRAM chip such as the one that's on the L-Star Plus board.
I can do this with RAM too, but the Propeller only has 32KB of hub memory available, part of which is used up by the program.
APPLE2 6502 EMULATOR TEXT BASED CODE
To emulate a ROM, all I need to do in Spin is to put a "file" command in the source code to load a ROM image file and then run a Memory module to map it into 6502 address space. In the projects that I've already written for L-Star, I've extensively used the Memory module which give the 6502 access to a memory area in the hub of the Propeller. Later models added a CRT controller chip to simplify the video hardware. There were 3 simple I/O chips in the early models: one for the keyboard and the cassette interface, one for the IEEE-488 (GPIB) interface, and one for the second cassette port and the user port. The early PET/CBM machines had up to 32KB RAM on board, a character-based memory-mapped video display, 8KB ROM for Microsoft BASIC, 2K ROM for the screen-based editor, and 4KB for the operating system which Commodore called the "Kernal". As you can see in the picture, I have a long way to go. And now I've finally made a start on writing a software-defined computer project to emulate the PET/CBM on L-Star. The PET-2001 was the first computer I ever used, when I was about 11 years old in 1978. If you've followed me long enough, you know that my original purpose for the #Propeddle: Software-Defined 6502 Computer project was to build a replica of the Commodore PET-2001 and CBM series computers.
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