BeagleBone 6502 RemoteProc cape

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BeagleBone 6502 RemoteProc cape

The BeagleBone 6502 RemoteProc cape is a prototype project that interfaces a WDC65C02S microprocessor to the BeagleBone host. The AM335x SoC on the BeagleBone emulates 65xx bus cycles utilizing the Ti_AM33XX_PRUSSv2. The BeagleBone provides both clock and emulated RAM for the 6502 such that a complete 6502 computer is realized with only some bus transceivers needed for glue logic.

Hardware Design

The following diagram shows a high-level schematic of the b6502 cape. The number of PRU pins available are limited so bus cycles are sampled by the PRU asserting a series of octal bus transceiver enables. In a read cycle, address high, address low, and data out are enabled in sequence. For a write cycle, address high, address low, and data in are enabled in sequence. This allows the PRU to wait for a clock edge, sample the R/W signal, and then fetch the address and data for the read or write operation. To complete the cycle, the PRU reads or writes to its local SRAM, providing the data to the 6502 on a read cycle or storing the data on a write cycle.

BeagleBone 6502 Schematic

A Fritzing diagram illustrates the wiring of the prototype on a breadboard.

BeagleBone 6502 Fritzing

The Fritzing project is here


  • b6502 3.6 Kernel branch is the kernel support for the Beaglebone, PRUSS, RemoteProc driver for the attached 6502, and a simple shared memory serial driver for communication with the 6502.
  • b6502 cape userspace tools contains PRU code to decode 6502 bus cycles and tools to create 6502 applications natively on the BeagleBone filesystem.


The PRU code requires that the user install the am335x PRU SW package from AM335x PRU Package. The 6502 examples require that the user install the *cc65* toolchain as documented at [1]. Only ca65/ld65 are required as there are currently no C examples.

PRU 6502 Bus Decoder

b6502_pruss is a C and PRU assembly tool that launches PRU firmware that will decode 65xx bus cycles providing access to AM335x-resident SRAM as local memory for the 6502 processor.


  • pokechar - simplest b6502rproc example. stores an ascii 'C' to sram
  • bvuart - echo characters back to the virtual uart kernel driver
  • bewoz - extended Woz Monitor with virtual uart support

Build and Run

This assumes GNU make/gcc and the compiled AM335x PRU SW package are installed on the BeagleBone rootfs.

root@bone2:~# cp am335x_pru_package/pru_sw/utils/pasm /usr/local/bin/
root@bone2:~# cp am335x_pru_package/pru_sw/app_loader/include/*.h /usr/local/include/
root@bone2:~# cp am335x_pru_package/pru_sw/app_loader/lib/* /usr/local/lib/
root@bone2:~# cd b6502/pruss/
root@bone2:~/b6502/pruss# make
gcc -I/usr/local/include -o b6502_pruss b6502_pruss.c -L/usr/local/lib -lprussdrv -lpthread
pasm -b b6502_pruss.p
PRU Assembler Version 0.80
Copyright (C) 2005-2012 by Texas Instruments Inc.
Pass 2 : 0 Error(s), 0 Warning(s)
Writing Code Image of 107 word(s)

This assumes GNU make and ca65/ld65 are installed on the BeagleBone rootfs and in the path. In addition, the b6502 pruss loader and kernel with b6502remoteproc and uio_pruss support plus the necessary DTS data must be present.

root@bone2:~/b6502/examples/pokechar# make
ca65 -l pokechar.lst pokechar.s
ld65 -o pokechar.bin -C b6502.cfg pokechar.o
root@bone2:~/b6502/examples/pokechar# cp pokechar.bin /lib/firmware/bone-6502-fw.bin

Now run it.

root@bone2:~/b6502/examples/pokechar# b6502_pruss
root@bone2:~/b6502/examples/pokechar# echo 1 > /sys/class/rproc/remoteproc0/on_off

Verify it executed as expected.

root@bone2:~/b6502/examples/pokechar# devmem2 0x4a31fff0 w
/dev/mem opened.
Memory mapped at address 0xb6f92000.
Read at address  0x4A311FF0 (0xb6f92ff0): 0x00000043