Minimal viable kernel

.gitignore

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+*~
+_*
+*.o
+*.d
+*.asm
+*.sym
+*.img
+*.elf
+vectors.S
+bootblock
+entryother
+initcode
+initcode.out
+kernelmemfs
+mkfs
+kernel/kernel
+user/usys.S
+.gdbinit
+.vscode
+.cache
+compile_commands.json
+tags

Makefile

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+TOOLPREFIX = riscv-none-elf
+
+CC = $(TOOLPREFIX)-gcc
+AS = $(TOOLPREFIX)-as
+LD = $(TOOLPREFIX)-ld
+OBJCOPY = $(TOOLPREFIX)-objcopy
+OBJDUMP = $(TOOLPREFIX)-objdump
+
+ASFLAGS = -march=rv64gc -mabi=lp64
+
+LDFLAGS = -Tlink.ld
+LDFLAGS += -m elf64lriscv
+
+CFLAGS = -Wall -Werror -O
+CFLAGS += -mcmodel=medany
+CFLAGS += -march=rv64gc -mabi=lp64
+CFLAGS += -ffreestanding -fno-common -nostdlib -mno-relax
+
+CFLAGS += -I.
+
+CFLAGS += -fno-stack-protector # Prevents code that needs libc / runtime support
+CFLAGS += -MD # Generate header dependency files (.d)
+CFLAGS += -fno-pie -no-pie # Fixed address linking
+CFLAGS += -ggdb -gdwarf-2 # GDB debug info
+CFLAGS += -fno-omit-frame-pointer # More reliable backtraces in GDB
+
+all: kernel.elf
+
+kernel.elf: entry.o start.o
+	@echo LD $@
+	@$(LD) $(LDFLAGS) -o $@ $^
+
+%.o: %.c
+	@echo CC $@
+	@$(CC) $(CFLAGS) -nostdinc -I. -c $< -o $@
+
+%.o: %.S
+	@echo AS $@
+	@$(AS) $(ASFLAGS) -o $@ $<
+
+qemu: kernel.elf
+	@echo QEMU $@
+	@qemu-system-riscv64 -machine virt -bios none -nographic -kernel kernel.elf
+
+clean:
+	rm -f *.o *.elf *.d
+
+-include *.d

entry.S

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+.section .text
+.globl _start
+_start:
+    li x1, 0x0
+    li x2, 0x0
+    li x3, 0x0
+    li x4, 0x0
+    li x5, 0x0
+    li x6, 0x0
+    li x7, 0x0
+    li x8, 0x0
+    li x9, 0x0
+    li x10, 0x0
+    li x11, 0x0
+    li x12, 0x0
+    li x13, 0x0
+    li x14, 0x0
+    li x15, 0x0
+    li x16, 0x0
+    li x17, 0x0
+    li x18, 0x0
+    li x19, 0x0
+    li x20, 0x0
+    li x21, 0x0
+    li x22, 0x0
+    li x23, 0x0
+    li x24, 0x0
+    li x25, 0x0
+    li x26, 0x0
+    li x27, 0x0
+    li x28, 0x0
+    li x29, 0x0
+    li x30, 0x0
+    li x31, 0x0
+
+    li t0, 0x10000000    # UART base address
+    li t1, 'E'           # Character to print
+    sb t1, 0(t0)
+    li t1, 'n'
+    sb t1, 0(t0)
+    li t1, 't'
+    sb t1, 0(t0)
+    li t1, 'r'
+    sb t1, 0(t0)
+    li t1, 'y'
+    sb t1, 0(t0)
+    li t1, '\n'
+    sb t1, 0(t0)
+
+    # Set up a stack for C.
+    la sp, stack0
+    li a0, 1024*4       # a0 = 4096
+    csrr a1, mhartid    # a1 = hart id
+    addi a1, a1, 1      # hartid + 1
+    mul a0, a0, a1      # a0 *= hartid+1
+    add sp, sp, a0      # sp += a0
+
+    # Jump to start() in start.c
+    call start
+
+1:
+    j 1b                 # Infinite loop

link.ld

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+ENTRY(_start)
+
+SECTIONS {
+  . = 0x80000000;
+
+  .text : {
+    *(.text)
+  } :text
+}
+
+PHDRS {
+  text PT_LOAD FLAGS(0x5); /* R + X */
+}
+

start.c

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+#define NCPU 3
+
+#define UART ((char *)0x10000000)
+
+void uart_putc(char c) {
+    *UART = c;
+}
+
+void uart_puts(const char *s) {
+    while (*s) {
+        uart_putc(*s++);
+    }
+}
+
+// Entry.S needs one stack per CPU.
+__attribute__((aligned(16))) char stack0[4096 * NCPU];
+
+void start() {
+    uart_puts("Hello Neptune!\n");
+}