README.md

RCPU_FPGA

RCPU_FPGA is an FPGA port of RCPU. This means that you can now run RCPU on actual hardware!

Toolchain installation

For Arch Linux: install icestorm-git, yosys-git and either arachne-pnr-git or nextpnr-git from the AUR.

You can then upload the code to the FPGA by running make rcpu_icestick or make rcpu_icestick_nextnpr in the rcpu/icestorm directory. You can choose what ROM will run on boot by editing the $readmemh argument in rcpu/icestorm/rcpu_icestick.v.

Limitations/differences from the RCPU emulator

• Memory is only backed by physical memory from 0 - 4095. After that, it wraps around. So the value in address 0 will always be the same value as the one in 4096, 8192, ...

• There is only enough room for 10 values on the stack. If you add any more, the values that were first pushed will be replaced by a dummy value.

• The multiply and divide ATH instructions are not implemented because they are too expensive.

• The arguments for the SYS instruction work different than in the emulator, see below.

SYS addresses

A SYS instruction expects an address on the top of the stack, and a value to write if the address indicates writing. Here is a table of devices and the address they correspond to. The 0-th bit is the most significant. The 14-th bit indicates reading, the 15-th bit indicates writing.

/*        bit   mode    device
          0     r/w     PMOD GPIO
          1     r/w     PMOD direction
          2     r/w     LEDS
          3     r/w     misc.out
          4     r/w     HDR1 GPIO
          5     r/w     HDR1 direction
          6     r/w     HDR2 GPIO
          7     r/w     HDR2 direction
          11      w     sb_warmboot
          12    r/w     UART RX, UART TX
          13    r       misc.in
*/

For example, if we wanted to turn on the LEDs, we would first push 0xFFFF (all bits are 1, so all leds will turn on). We would then push 0b0010_0000_0000_0001 to indicate we want to write to the LEDs (bit 2 and 15 are set). After that, we can use the SYS instruction to turn the leds on.

If we wanted to read the LEDs, we would push 0b0010_0000_0000_0010 and then use the SYS instruction. The value of the LEDs will then be on the stack.

Example assembled programs

The example ROMs in rcpu/build/*.hex are built with the RCPU assembler. The sourcecode of these are available in the main RCPU repository.

Bootloader

The default ROM that runs is a simple bootloader: it reads addresses and values from UART and stores them in RAM. This allows you to load your programs without having to recompile the FPGA code and having to flash it each time you want to change your program

Questions?

Please don't hesitate to open an issue/contact me

Roadmap

• Get UART tx working
• Get UART rx working
• Write some code in RCPU assembly that allows uploading code without reflashing