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# Zephyr Port

The FWRISC Zephyr port consists of SoC and Board files derived from the relevant files for Pulpino and risc-privilege.

The only somewhat-unique aspect of porting Zephyr to FWRISC is that FWRISC does not (currently) support interrupts.

As a consequence, measuring time is done by polling the MCYCLE CSR instead of waiting for timer interrupts to occur.

This also means that Zephyr does not currently support truly-preemptive multitasking.

## Porting Tasks

### Creating a dummy Timer Driver

Zephyr appears to assume that a timer driver exists. Consequently, a stub fwrisc_timer driver (drivers/timer/fwrisc_timer.c)

was created. This is a completely-NOP implementation.

### Implementing the Idle Routine

Zephyr applications still need to tell time, so we need an alternative to a timer interrupt. The FWRISC Zephyr port

enables keeping time via an implementation of the k_cpu_idle function that issues a clock event every N

MCYCLE counts.

```

void k_cpu_idle(void)

{

        u32_t mcycle;

        while (1) {

                __asm__ volatile("csrr %0, mcycle" : "=r"(mcycle));

                if ((mcycle-last_mcycle) >= CYCLES_PER_TICK) {

                        z_clock_announce(1);

                        last_mcycle = mcycle;

                        break;

                }

        }

        // Make a system call to trigger thread rescheduling

        __asm__ volatile("ecall");

}

```

### Boards

The FWRISC port of Zephyr defines two boards:

- fwrisc_sim -- A board definition for use with RTL simulation targets. Uses the RAM console, and defines the clock frequency so as to shorten simulation runs.

- fwrisc_fpga -- A board definition for use with FPGA platforms. Uses a UART console and defines the clock frequency appropriately