nixos-riscv/bootrom/rbootrom.S

/*
 * Copyright (c) 2024, Barcelona Supercomputing Center (BSC)
 * SPDX-License-Identifier: MIT
 *
 * RBOOTROM v1.0
 * Modified by Rodrigo Arias Mallo <rodrigo.arias@bsc.es>
 *
 * This is a custom bootrom that prints some information to the UART when
 * starting, as well as when it hangs. It assumes the UART is at UART_BASE and
 * it will jump to DRAM_BASE to continue the boot.
 */

#define DRAM_BASE 0x80000000
#define UART_BASE 0x40001000

.macro PUTC, ch
	li a0, \ch
	jal putchar
.endm

.section .text.start, "ax", @progbits
.globl _start
_start:
	jal print_hello		// Print initial message on HART 0 only

	la t0, _hang
	csrw mtvec, t0		// Set the machine trap vector

	csrr a0, mhartid	// Load HART ID into a0
	li s0, DRAM_BASE	// Load next address into s0
	jr s0			// Jump to s0

.section .text.hang, "ax", @progbits
.globl _hang
_hang:

	PUTC '\n'
	PUTC '\r'
	PUTC 'R'
	PUTC 'B'
	PUTC 'O'
	PUTC 'O'
	PUTC 'T'
	PUTC 'R'
	PUTC 'O'
	PUTC 'M'
	PUTC ' '
	PUTC 'H'
	PUTC 'A'
	PUTC 'N'
	PUTC 'G'
	PUTC '\n'
	PUTC '\r'

	/* Hang */
	csrr a0, mhartid
1:
	wfi
	j 1b

.section .text, "ax", @progbits

putchar:
	// Wait a bit before writing to UART
	li t0, 100000		// Delay a bit
1:
	addi t0, t0, -1		// Decrement
	bne  t0, zero, 1b	// Repeat if not zero

	// Write the character now
	la t0, UART_BASE	// Transmitter Holding Buffer
	sb a0, 0(t0)		// Write a0 byte
	ret

print_hello:
	csrr t0, mhartid	// Load HART ID into a0
	beq  t0, zero, 1f	// Print message on HART 0 only
	ret
1:
	mv s0, ra		// Save return address
	PUTC '\n'		// Identify bootroom
	PUTC '\r'
	PUTC 'R'
	PUTC 'B'
	PUTC 'O'
	PUTC 'O'
	PUTC 'T'
	PUTC 'R'
	PUTC 'O'
	PUTC 'M'
	PUTC ' '
	PUTC 'v'
	PUTC '1'
	PUTC '.'
	PUTC '0'
	PUTC ' '
	PUTC ':'
	PUTC '^'
	PUTC ')'
	PUTC '\n'
	PUTC '\r'

	// Print jumping address
	PUTC 'J'
	PUTC 'u'
	PUTC 'm'
	PUTC 'p'
	PUTC 'i'
	PUTC 'n'
	PUTC 'g'
	PUTC ' '
	PUTC 't'
	PUTC 'o'
	PUTC ' '
	PUTC '0' // TODO: Compute from DRAM_BASE
	PUTC 'x'
	PUTC '8'
	PUTC '0'
	PUTC '0'
	PUTC '0'
	PUTC '_'
	PUTC '0'
	PUTC '0'
	PUTC '0'
	PUTC '0'
	PUTC '.'
	PUTC '.'
	PUTC '.'
	PUTC '\n'
	PUTC '\r'

	mv ra, s0		// Restore return address
	ret