04-Bare-Metal-C-Language

04: Bare Metal Hello RISC-V - C Language Version

Background

C Program and Stack

The most important thing we need to know is that C programs rely Stack to run.

• For RISC-V, the stack pointer register is sp or x2.
• When a C function is called, it allocates memory for its running by decreasing sp.

The default sp at startup is 0x0, and the memory space below 0x0 is of course illegal.

Therefore, we must set a reasonable value for sp, so that we can jump from our startup code written in assembly to a C function.

C Program/RISC-V Calling Convention

Setuping stack pointer is enough for jumping from our assembly startup code to a C function.
However, to call an assembly code snippet from a C function and return it, we need to learn how a C function passes parameters and receives a return value.

Just a reminder, we can ignore this for now in this tutorial. For deatils, refer to RISC-V Calling Conventions.

Hello, RISC-V!

Step.1 Setup Stack Pointer for C Program

Our firmware will be loaded to 0x80000000, we can give sp a proper value for C program.

.global _clang_env_init

.text
_clang_env_init:
li sp, 0x80200000 # give a proper memory address value to stack pointer
j _uart_init # jump to a C function

Step.2 Write UART initialization code with C Language

typedef char u8;
typedef short u16;
typedef int u32;
typedef long u64;

#define UART_BASE 0x10000000

#define UART_RBR_OFFSET		0	/* In:  Recieve Buffer Register */
#define UART_THR_OFFSET		0	/* Out: Transmitter Holding Register */
#define UART_DLL_OFFSET		0	/* Out: Divisor Latch Low */
#define UART_IER_OFFSET		1	/* I/O: Interrupt Enable Register */
#define UART_DLM_OFFSET		1	/* Out: Divisor Latch High */
#define UART_FCR_OFFSET		2	/* Out: FIFO Control Register */
#define UART_IIR_OFFSET		2	/* I/O: Interrupt Identification Register */
#define UART_LCR_OFFSET		3	/* Out: Line Control Register */
#define UART_MCR_OFFSET		4	/* Out: Modem Control Register */
#define UART_LSR_OFFSET		5	/* In:  Line Status Register */
#define UART_MSR_OFFSET		6	/* In:  Modem Status Register */
#define UART_SCR_OFFSET		7	/* I/O: Scratch Register */
#define UART_MDR1_OFFSET	8	/* I/O:  Mode Register */

#define PLATFORM_UART_INPUT_FREQ 10000000
#define PLATFORM_UART_BAUDRATE 115200

static u8 *uart_base_addr = (u8 *)UART_BASE;

static inline void writeb(u8 val, volatile void *addr)
{
	asm volatile("sb %0, 0(%1)" : : "r"(val), "r"(addr));
}

static inline u8 readb(const volatile void *addr)
{
	u8 val;
	asm volatile("lb %0, 0(%1)" : "=r"(val) : "r"(addr));
	return val;
}

static void set_reg(u32 offset, u32 val)
{
	writeb(val, uart_base_addr + offset);
}

static u32 get_reg(u32 offset)
{
	return readb(uart_base_addr + offset);
}

static void uart_putc(u8 ch) {
  set_reg(UART_THR_OFFSET, ch);
}

static void uart_print(char *str) {
  while (*str) {
    uart_putc(*str++);
  }
}

int _uart_init() {
  u16 bdiv = (PLATFORM_UART_INPUT_FREQ + 8 * PLATFORM_UART_BAUDRATE) / (16 * PLATFORM_UART_BAUDRATE);

  /* Disable all interrupts */
	set_reg(UART_IER_OFFSET, 0x00);
	/* Enable DLAB */
	set_reg(UART_LCR_OFFSET, 0x80);

  if (bdiv) {
		/* Set divisor low byte */
		set_reg(UART_DLL_OFFSET, bdiv & 0xff);
		/* Set divisor high byte */
		set_reg(UART_DLM_OFFSET, (bdiv >> 8) & 0xff);
	}

	/* 8 bits, no parity, one stop bit */
	set_reg(UART_LCR_OFFSET, 0x03);
	/* Enable FIFO */
	set_reg(UART_FCR_OFFSET, 0x01);
	/* No modem control DTR RTS */
	set_reg(UART_MCR_OFFSET, 0x00);
	/* Clear line status */
	get_reg(UART_LSR_OFFSET);
	/* Read receive buffer */
	get_reg(UART_RBR_OFFSET);
	/* Set scratchpad */
	set_reg(UART_SCR_OFFSET, 0x00);
  char *str = "Hello, RISC-V!\n";
  uart_print(str);

  while (1) {}
}

Step.3 Build and Run

IMPORTANT: Set Proper Location Counter in Linker Script!!!

make run

Read from console: echo

We can also read from console/uart, then print what user enters.

The complete code is in echo.c. Run with make echo.

/* ... */
static int readble() {
	return get_reg(UART_LSR_OFFSET) & UART_LSR_DR;
}

/* ... */

int _uart_init() {
	/* ... */
	uart_print("Hello, RISC-V!\n");
	uart_print("echo > ");
	
	while (1) {
		if (readble()) {
			int ch = uart_getc();
			uart_putc(ch);
		}
	}
	/* ...... */
}

References

• RISC-V Calling Conventions
• RISC-V Toolchain Conventions
• RISC-V ELF psABI Document
• RISC-V Assembly Programmer's Manual