trashernet-soc/sw/bootrom/main.c

#include <inttypes.h>

#define UART0_BASE 0x81000000
#define UART0_SR (*((volatile uint32_t *) (UART0_BASE + 0x00)))
#define UART0_DR (*((volatile uint32_t *) (UART0_BASE + 0x04)))

#define UART0_SR_RX_DATA_EMPTY (1 << 0)
#define UART0_SR_TX_FULL (1 << 1)

/*
// Set up stack pointer
asm("_start:\
	xor sp, sp, sp;\
	lui sp, 0x80100;\
	j main;\
");
*/

__attribute__((noreturn)) void _start() {
	uint8_t state = 0;
	uint8_t c;
	uint8_t opcode;
	uint8_t *ptr;
	uint32_t length;
	uint32_t sr;

	/*while (1) {
		while (UART0_SR & UART0_SR_RX_DATA_EMPTY);
		c = UART0_DR;
		while (UART0_SR & UART0_SR_TX_FULL);
		UART0_DR = c;
	}*/

	while (1) {
		while (UART0_SR & UART0_SR_RX_DATA_EMPTY);
		c = (uint8_t)UART0_DR;
		sr = (sr << 8) | c;
		state++;
		switch (state) {
			case 1: // Opcode
				opcode = c;
				if (c == 0) // NOP
					state = 0;
				break;
			case 5: // Address
				ptr = (uint8_t*)sr;
				if (opcode == 3) { // Jump
					//((void (*)()) ptr)();
					asm("jalr %0" : : "r"(ptr)); // Not sure why, but the jump above causes GCC to save variables to the stack at the beginning of this function
					__builtin_unreachable();
				}
				break;
			case 9: // Data
				length = sr;
				if (opcode == 2) { // Read
					for (; length > 0; length--) {
						while (UART0_SR & UART0_SR_TX_FULL);
						UART0_DR = *(ptr++);
					}
				} else { // Write
					for (; length > 0; length--) {
				                while (UART0_SR & UART0_SR_RX_DATA_EMPTY);
						*(ptr++) = UART0_DR;
					}
				}
				state = 0;
				break;
			default:
				break;
		}
	}
	__builtin_unreachable();
}