Add RISC-V kernel example

examples/kernel-multiboot2-grub/linker.ld

@@ -1,7 +1,7 @@
 OUTPUT_ARCH("i386")
 ENTRY(_start)
 
-_kernel_offset = 0xC0000000; /* 3 GB */
+_kernel_offset = 0xC0000000; /* 3 GiB */
 
 _kernel_phys_base = 4M;
 _kernel_virt_base = (_kernel_phys_base + _kernel_offset);

examples/kernel-multiboot2-limine/linker.ld

@@ -1,7 +1,7 @@
 OUTPUT_ARCH("i386")
 ENTRY(_start)
 
-_kernel_offset = 0xC0000000; /* 3 GB */
+_kernel_offset = 0xC0000000; /* 3 GiB */
 
 _kernel_phys_base = 4M;
 _kernel_virt_base = (_kernel_phys_base + _kernel_offset);

examples/kernel-riscv64/.gitignore

@@ -0,0 +1 @@
+/kernel.elf

examples/kernel-riscv64/Makefile

@@ -7,5 +7,31 @@ CC = $(CCPREFIX)gcc
 
 QEMU = qemu-system-riscv64
 
-run:
-	$(QEMU) -machine virt -serial stdio
+KERNEL = kernel.elf
+LINKERSCR = linker.ld
+
+CFLAGS = \
+	-std=c99        \
+	-pedantic       \
+	-Wall           \
+	-Wextra         \
+	-Werror         \
+	-ffreestanding  \
+	-mcmodel=medany
+
+OBJS = main.c.o start.S.o
+
+run: $(KERNEL)
+	$(QEMU) -machine virt -bios $< -serial stdio -display none
+
+clean:
+	rm -f $(KERNEL) $(OBJS)
+
+$(KERNEL): $(LINKERSCR) $(OBJS)
+	$(CC) -T $(LINKERSCR) -o $@ $(OBJS) -nostdlib -lgcc
+
+%.c.o: %.c
+	$(CC) -c $< -o $@ $(CFLAGS)
+
+%.S.o: %.S
+	$(AS) $< -o $@

examples/kernel-riscv64/linker.ld

@@ -0,0 +1,26 @@
+ENTRY(_start);
+
+. = 0x80000000;
+
+SECTIONS {
+    /* Include entry point at start of binary */
+    .text : ALIGN(4K) {
+        *(.init);
+        *(.text);
+    }
+    .bss : ALIGN(4K) {
+        PROVIDE(bss_start = .);
+        *(.bss);
+        . += 4096;
+        PROVIDE(stack_top = .);
+        . += 4096;
+        PROVIDE(global_pointer = .);
+        PROVIDE(bss_end = .);
+    }
+    .rodata : ALIGN(4K) {
+        *(.rodata);
+    }
+    .data : ALIGN(4K) {
+        *(.data);
+    }
+}

examples/kernel-riscv64/main.c

@@ -0,0 +1,19 @@
+#include <stddef.h>
+#include <stdint.h>
+
+static unsigned char *const uart = (unsigned char*)0x10000000;
+
+static void putchar(char c) {
+    *uart = c;
+}
+
+static void print(const char * str) {
+    while (*str != '\0') {
+        putchar(*str);
+        str++;
+    }
+}
+
+void main() {
+    print("Hello world!\r\n");
+}

examples/kernel-riscv64/start.S

@@ -0,0 +1,37 @@
+.section .init
+
+.option norvc
+
+.type _start, @function
+.global _start
+_start:
+    .cfi_startproc
+
+.option push
+.option norelax
+    la gp, global_pointer
+.option pop
+
+    /* Reset satp */
+    csrw satp, zero
+
+    /* Setup stack */
+    la sp, stack_top
+
+    /* Clear the BSS section */
+    la t5, bss_start
+    la t6, bss_end
+bss_clear:
+    sd zero, (t5)
+    addi t5, t5, 8
+    bgeu t5, t6, bss_clear
+
+    la t0, main
+    csrw mepc, t0
+
+    /* Jump to kernel! */
+    tail main
+
+    .cfi_endproc
+
+.end