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mballance
# Zephyr Port
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The FWRISC Zephyr port consists of SoC and Board files derived from the relevant files for Pulpino and risc-privilege.
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The only somewhat-unique aspect of porting Zephyr to FWRISC is that FWRISC does not (currently) support interrupts.
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As a consequence, measuring time is done by polling the MCYCLE CSR instead of waiting for timer interrupts to occur.
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This also means that Zephyr does not currently support truly-preemptive multitasking.
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## Porting Tasks
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### Creating a dummy Timer Driver
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Zephyr appears to assume that a timer driver exists. Consequently, a stub fwrisc_timer driver (drivers/timer/fwrisc_timer.c)
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was created. This is a completely-NOP implementation.
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### Implementing the Idle Routine
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Zephyr applications still need to tell time, so we need an alternative to a timer interrupt. The FWRISC Zephyr port
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enables keeping time via an implementation of the k_cpu_idle function that issues a clock event every N
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MCYCLE counts.
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```
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void k_cpu_idle(void)
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{
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u32_t mcycle;
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while (1) {
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__asm__ volatile("csrr %0, mcycle" : "=r"(mcycle));
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if ((mcycle-last_mcycle) >= CYCLES_PER_TICK) {
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z_clock_announce(1);
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last_mcycle = mcycle;
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break;
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}
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}
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// Make a system call to trigger thread rescheduling
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__asm__ volatile("ecall");
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}
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```
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### Boards
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The FWRISC port of Zephyr defines two boards:
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- 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.
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- fwrisc_fpga -- A board definition for use with FPGA platforms. Uses a UART console and defines the clock frequency appropriately
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