generate BootROM contents from assembly code

bootrom/.gitignore

@@ -0,0 +1 @@
+*.elf

bootrom/Makefile

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

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 debugger to load a program and change the PC.
+  j _start // reset vector
+  .word 0 // reserved
+  .word 0 // reserved
+  .word 0 // pointer to config string
+  .word 0 // default trap vector
+  .word 0
+  .word 0
+  .word 0

bootrom/linker.ld

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