Synchronous Exception Handler

目前在 sync_exc_router 會透過 ESR_EL1 帶的 EC 與 ISS 決定執行哪個 handler，目前先簡易的實作一個可以動的 System call，之後會再將 System call number 改為透過正常的參數傳遞。

exception.c

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void sync_exc_router(unsigned long esr, unsigned long elr) {
    int ec = (esr >> 26) & 0b111111;
    int iss = esr & 0x1FFFFFF;
    if (ec == 0b010101) {  // system call
        switch (iss) {
            case 1:
                uart_printf("Exception return address 0x%x\n", elr);
                uart_printf("Exception class (EC) 0x%x\n", ec);
                uart_printf("Instruction specific syndrome (ISS) 0x%x\n", iss);
                break;
            case 2:
                arm_core_timer_enable();
                arm_local_timer_enable();
                break;
            case 3:
                arm_core_timer_disable();
                arm_local_timer_disable();
                break;
            case 4:
                asm volatile ("mrs %0, cntfrq_el0" : "=r" (cntfrq_el0)); // get current counter frequency
                asm volatile ("mrs %0, cntpct_el0" : "=r" (cntpct_el0)); // read current counter
                break;
        }
    }
    else {
        uart_printf("Exception return address 0x%x\n", elr);
        uart_printf("Exception class (EC) 0x%x\n", ec);
        uart_printf("Instruction specific syndrome (ISS) 0x%x\n", iss);
    }
}

shell.c

按照 spec 要求實作 exc 指令，用於測試 Synchronous Exception

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void shell_controller(char* cmd) {
    ...
    else if (!strcmp(cmd, "exc")) {
        asm volatile("svc #1");
    }
    ...
}

util.c

因為 shell 現在執行在 EL0，不能直接碰到 System Registers，所以 timestamp 指令也需要透過 system call 下去拿到頻率與 counter 再進行相除，正常來說應該要把一個指標當作參數傳進 system call 拿到對應的值，但目前還沒有這麼高級。

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float get_timestamp() {
    asm volatile ("svc #4");
    return (float) cntpct_el0 / cntfrq_el0;
}

More details: ESR_EL1

Holds syndrome information for an exception taken to EL1.

[63:32]	[31:26]	[25]	[24:0]
Reserved	EC	IL	ISS

EC: Exception Class. Indicates the reason for the exception that this register holds information about.
IL: Instruction Length for synchronous exceptions.
ISS: Instruction Specific Syndrome.

When EC = 0b000000 (exception with an unknown reason), ISS is not valid