Interesting: http://hackaday.com/2016/12/19/portable-apple-ii-on-an-avr/

I saw that on Hackaday and was very impressed. He also has a nice web page online about the project, and the performance seems to be pretty decent judging from his video.

Apple II's were so expensive back in the day that I never even saw one until 1985 or so, and that one was running UCSD Pascal at my university; I've been wondering how hard it would be to emulate one with my L-Star project, which uses a Propeller and a real 6502, and this gives me hope that emulation should be possible with little or no additional hardware.

===Jac

Wow! I have an Uzebox and the AVR 1284 is a capable chip, but that has to be pushing it to the limits.

I would think that the l-star would be in good shape to do a similar emulation because most of the 1284's horse power is being used for the system RAM and 6502 emulation. The l-star offload both of those and uses a PC keyboard which is low overhead compared to keyboard matrix scanning.

I started reading the thesis - it looks like it should be very good, discussing different approaches to emulation and how to get performance on such a platform.
http://maxstrauch.github.io/projects/bs ... index.html
http://maxstrauch.github.io/projects/bs ... _final.pdf

Yup! Unfortunately the L-Star sacrifices a lot of pins on the Propeller just to get things to work. It even uses the I2C clock as clock output to the 6502 (while holding the data line high so the EEPROM that has the Propeller firmware doesn't get activated). This leaves only 3 pins for I/O, or 2 pins if you're using the SRAM chip, or 1 pin if you want to use more than 64K of the SRAM chip!

I have a vague plan to improve the next generation of the L-Star with an additional microcontroller (probably a DIP PIC with a USB interface) as a sort-of intelligent I/O extender. It would probably be connected to all Propeller pins P25-P31 and do things like serial-to-USB translation (currently done by the Prop Plug), PS/2 keyboard to serial translation, interfacing with an SD card, I2C based I/O extenders, and maybe an SPI based LCD color display. Sounds easy, right?

===Jac

A also took a look at this project. The major enhancement to other AVR based 6502 emulators is the integration of the screen and some software added to the emulator that captures writes to addresses of the RAM that correspond to the apple text/lores screen. Here depending on the softswitches he translates this to a character or bitmap write to the screen. That is the part I'm really interested in. However I did not find the source code. As we have seen from other emulators (8-bit and Klaus2m5 cores of their 65C02 emulator) we can expect decent performance with a 20MHz clocked AVR. Somewhere between 1 and 2 MHz.

The keyboard actually is not scanned by the Atmega1284 but by a dedicated Mega8 which translates the keystrokes to a character that is then sent via the UART (see signal TX_S) to the main AVR. So keyboard does not consume any CPU on the Atmega1284. It is on par with a PS/2 or serial interface.

L-Star as I understand takes a completely different approach and has a dedicated 6502 and RAM and the propeller acts more like the super-all-inclusive-glue. So it is difficult to compare those projects.

I also would say adding more memory to this project is not as easy as it seems. The AVR is very bad in accessing external memory. You really need to think about external memory right from the beginning when designing an emulator as in Klaus version. I'd say that it would be easier to integrate the memory mapped Apple II screen to the emulator of Klaus than adding more memory to this project.