3 files changed, 95 insertions, 0 deletions
diff --git a/boot/amd64/generic/biosboot/Makefile b/boot/amd64/generic/biosboot/Makefile
new file mode 100644
index 0000000..e9854be
--- /dev/null
+++ b/boot/amd64/generic/biosboot/Makefile
@@ -0,0 +1,15 @@
+#
+# Copyright (c) 2026, Apollo Telephone Laboratories.
+# Provided under the BSD-3 clause.
+#
+
+include ../mk/build.mk
+
+CFLAGS = $(INTERNAL_CFLAGS)
+
+.PHONY: all
+all: biosboot
+
+.PHONY: biosboot
+biosboot:
+	$(CC) $(CFLAGS) -Tlink.ld biosboot.S -o biosboot.bin
diff --git a/boot/amd64/generic/biosboot/biosboot.S b/boot/amd64/generic/biosboot/biosboot.S
new file mode 100644
index 0000000..bf2b004
--- /dev/null
+++ b/boot/amd64/generic/biosboot/biosboot.S
@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2026, Apollo Telephone Laboratories.
+ * Provided under the BSD-3 clause.
+ */
+
+    .text
+    .code16
+    .globl _start
+_start:
+    cli
+    xor %ax, %ax
+    mov %ax, %ds
+    mov %ax, %es
+    mov %ax, %ss
+
+    lgdt gdtr
+    mov %cr0, %eax
+    or $1, %eax
+    mov %eax, %cr0
+    ljmp $0x08,$1f
+
+    .code32
+1:  mov $0x10, %ax
+    mov %ax, %es
+    mov %ax, %ss
+    mov %ax, %fs
+    mov %ax, %gs
+    mov %ax, %ds
+    jmp c_thunk
+
+gdt:
+.null:
+    .quad 0
+.code:
+    .word 0xFFFF
+    .word 0x0000
+    .byte 0x00
+    .byte 0b10011010
+    .byte 0b11001111
+    .byte 0x00
+.data:
+    .word 0xFFFF
+    .word 0x0000
+    .byte 0x00
+    .byte 0b10010010
+    .byte 0b11001111
+    .byte 0x00
+gdtr:
+    .word . - gdt - 1
+    .long gdt
+
+/* Fall-through : everything above this */
+c_thunk:
+    nop
diff --git a/boot/amd64/generic/biosboot/link.ld b/boot/amd64/generic/biosboot/link.ld
new file mode 100644
index 0000000..f039b2b
--- /dev/null
+++ b/boot/amd64/generic/biosboot/link.ld
@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2026, Apollo Telephone Laboratories.
+ * Provided under the BSD-3 clause.
+ */
+
+OUTPUT_FORMAT(binary)
+
+SECTIONS {
+    . = 0x7C00;
+
+    .text : {
+        *(.text)
+    }
+
+    .data : {
+        *(.data)
+    }
+
+    .rodata : {
+        *(.rodata)
+    }
+
+    .bss : {
+        *(.bss)
+    }
+}