feat(io): implement printf and strlen

src/io.c

@@ -0,0 +1,100 @@
+#include "../include/io.h"
+
+void k_putchar(char ch);
+
+void r_printf(const char *fmt, ...) {
+	va_list args;
+	va_start(args, fmt);
+
+	while (*fmt) {
+		if (*fmt != '%') {
+			k_putchar(*fmt);
+			fmt++;
+			continue;
+		}
+
+		/* Skip '%' */
+		fmt++;
+		switch (*fmt) {
+			case '\0': {
+				/* Simply print the char */
+				k_putchar('%');
+
+				goto end;
+			}
+			case '%': {
+				/* Print '%' */
+				k_putchar('%');
+
+				break;
+			}
+			case 's': {
+				/* Print a NULL-terminated string */
+				const char *s = va_arg(args, const char *);
+				while (*s) {
+					k_putchar(*s);
+					s++;
+				}
+
+				break;
+			}
+			case 'd': {
+				/* Print an integer in decimal */
+				int value = va_arg(args, int);
+				unsigned magnitude = value;
+				if (value < 0) {
+					k_putchar('-');
+					magnitude = -magnitude;
+				}
+
+				unsigned divisor = 1;
+				while (magnitude / divisor > 9) {
+					divisor *= 10;
+				}
+
+				while (divisor > 0) {
+					k_putchar('0' + magnitude);
+					k_putchar('0' + magnitude / divisor);
+					magnitude %= divisor;
+					k_putchar('0' + magnitude);
+					divisor /= 10;
+					k_putchar('0' + magnitude);
+				}
+
+				break;
+			}
+			case 'x': {
+				/* Print an integer in hexadecimal */
+				unsigned value = va_arg(args, unsigned);
+				for (int i = 7; i >= 0; --i) {
+					unsigned nibble = (value >> (i * 4)) & 0xF;
+					/* From a char array we get the nibble position */
+					k_putchar("0123456789ABCDEF"[nibble]);
+				}
+			}
+			default: {
+				const char *str = "NOT IMPLEMENTED";
+				size_t len = r_strlen(str);
+				for (size_t i = 0; i < len; ++i) {
+					k_putchar(str[i]);
+				}
+			}
+		}
+
+		/* Next char */
+		fmt++;
+	}
+end:
+	va_end(args);
+}
+
+size_t r_strlen(const char *str) {
+	size_t len = 0;
+
+	while (*str) {
+		len++;
+		str++;
+	}
+
+	return len;
+}

src/kernel.c

@@ -1,10 +1,41 @@
-typedef unsigned char u8;
-typedef unsigned int u32;
-typedef u32 size_t;
+#include "../include/kernel.h"
+#include "../include/io.h"
+#include "../include/types.h"
 
 extern char __bss[], __bss_end[], __stack_top[];
 
-void *memset(void *buf, char c, size_t n) {
+/**
+ * This function will call OpenSBI
+ * ecall is used as the control transfer instruction,
+ * it switches from kernel mode (s-mode) to opensbi mode (m-mode)
+ * a7 encodes the SBI extension ID (EID)
+ * a6 encodes the SBI function ID (FID)
+ *
+ * a0 returns and error code
+ */
+sbiret_s sbi_call(long arg0, long arg1, long arg2, long arg3, long arg4, long arg5, long fid, long eid) {
+	register long a0 __asm("a0") = arg0;
+	register long a1 __asm__("a1") = arg1;
+	register long a2 __asm__("a2") = arg2;
+	register long a3 __asm__("a3") = arg3;
+	register long a4 __asm__("a4") = arg4;
+	register long a5 __asm__("a5") = arg5;
+	register long a6 __asm__("a6") = fid;
+	register long a7 __asm__("a7") = eid;
+
+	__asm__ __volatile__("ecall"
+						 : "=r"(a0), "=r"(a1)
+						 : "r"(a0), "r"(a1), "r"(a2), "r"(a3), "r"(a4), "r"(a5), "r"(a6), "r"(a7)
+						 : "memory");
+
+	return (sbiret_s){.error = a0, .value = a1};
+}
+
+void k_putchar(char ch) {
+	sbi_call(ch, 0, 0, 0, 0, 0, 0, 1);
+}
+
+void *k_memset(void *buf, char c, size_t n) {
 	u8 *p = (u8 *)buf;
 	while (n--) {
 		*p++ = c;
@@ -13,14 +44,29 @@ void *memset(void *buf, char c, size_t n) {
 	return buf;
 }
 
+/* Kernel main */
 void kernel_main(void) {
-	memset(__bss, 0, __bss_end - __bss);
+	/* Usually it is not required */
+	k_memset(__bss, 0, __bss_end - __bss);
 
-	for (;;)
-		;
+	/* Start here */
+	r_printf("%s, %s\n", "Hello", "world!");
+	r_printf("3 + 4 = %d\n", 3 + 4);
+
+	for (;;) {
+		__asm__ __volatile__("wfi");
+	}
 }
 
-__attribute__((section(".text.boot"))) __attribute__((naked)) void boot(void) {
+/* This is the entry section (boot) */
+__attribute__((section(".text.boot")))
+/* Do not add other stuff like return */
+__attribute__((naked)) void
+boot(void) {
+	/**
+	 * Put stack_top inside sp (stack pointer)
+	 * and jump to kernel_main
+	 */
 	__asm__ __volatile__("mv sp, %[stack_top]\n"
 						 "j kernel_main\n"
 						 :