initial commit

bootrom/.gitignore

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+*.elf
+*.dump

bootrom/Makefile

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+bootrom_img = bootrom.img
+
+GCC=riscv64-unknown-elf-gcc
+OBJCOPY=riscv64-unknown-elf-objcopy
+OBJDUMP=riscv64-unknown-elf-objdump
+
+all: $(bootrom_img)
+
+%.img: %.elf
+	$(OBJCOPY) -O binary --change-addresses=-0x1000 --only-section .text $< $@
+
+%.elf: %.S linker.ld
+	$(GCC) -Tlinker.ld $< -nostdlib -static -o $@
+
+%.dump: %.elf
+	$(OBJDUMP) -d $< > $@

bootrom/bootrom.S

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+.text
+.global _start
+_start:
+  // This boot ROM doesn't know about any boot devices, so it just spins,
+  // waiting for the serial interface to load the program and interrupt it
+  j setup_wfi_loop // reset vector
+  .word 0 // reserved
+  .word 0 // reserved
+  .word 0 // pointer to config string
+default_trap_vec:
+  j boot_trap // default trap vector
+  .word 0
+  .word 0
+  .word 0
+
+setup_wfi_loop:
+  la a0, default_trap_vec
+  csrw mtvec, a0
+  li a0, 8 // MIE or MSIP bit
+  csrw mie, a0 // set only MSIP in mie CSR
+  csrw mideleg, zero // no delegation
+  csrs mstatus, a0 // set MIE in mstatus CSR
+wfi_loop:
+  wfi
+  j wfi_loop
+
+boot_trap:
+  csrr a0, mhartid
+  sll a0, a0, 2 // offset for hart msip
+  li a1, 0x2000000 // base address of clint
+  add a0, a0, a1
+  sw zero, 0(a0) // clear the interrupt
+  li a0, 0x80000000 // program reset vector
+  csrw mepc, a0 // return from interrupt to start of user program
+  mret

bootrom/linker.ld

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+SECTIONS
+{
+    . = 0x1000;
+    .text : { *(.text) }
+}