74 lines
1.5 KiB
C
74 lines
1.5 KiB
C
#include <inttypes.h>
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#define UART0_BASE 0x81000000
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#define UART0_SR (*((volatile uint32_t *) (UART0_BASE + 0x00)))
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#define UART0_DR (*((volatile uint32_t *) (UART0_BASE + 0x04)))
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#define UART0_SR_RX_DATA_EMPTY (1 << 0)
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#define UART0_SR_TX_FULL (1 << 1)
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/*
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// Set up stack pointer
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asm("_start:\
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xor sp, sp, sp;\
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lui sp, 0x80100;\
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j main;\
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");
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*/
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__attribute__((noreturn)) void _start() {
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uint8_t state = 0;
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uint8_t c;
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uint8_t opcode;
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uint8_t *ptr;
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uint32_t length;
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uint32_t sr;
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/*while (1) {
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while (UART0_SR & UART0_SR_RX_DATA_EMPTY);
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c = UART0_DR;
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while (UART0_SR & UART0_SR_TX_FULL);
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UART0_DR = c;
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}*/
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while (1) {
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while (UART0_SR & UART0_SR_RX_DATA_EMPTY);
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c = (uint8_t)UART0_DR;
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sr = (sr << 8) | c;
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state++;
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switch (state) {
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case 1: // Opcode
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opcode = c;
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if (c == 0) // NOP
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state = 0;
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break;
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case 5: // Address
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ptr = (uint8_t*)sr;
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if (opcode == 3) { // Jump
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//((void (*)()) ptr)();
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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
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__builtin_unreachable();
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}
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break;
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case 9: // Data
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length = sr;
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if (opcode == 2) { // Read
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for (; length > 0; length--) {
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while (UART0_SR & UART0_SR_TX_FULL);
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UART0_DR = *(ptr++);
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}
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} else { // Write
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for (; length > 0; length--) {
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while (UART0_SR & UART0_SR_RX_DATA_EMPTY);
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*(ptr++) = UART0_DR;
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}
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}
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state = 0;
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break;
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default:
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break;
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}
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}
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__builtin_unreachable();
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}
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