From 3c90dd8b3d3ae54ad1243ca1ac9b523b53b5dff5 Mon Sep 17 00:00:00 2001
From: Paul Mathieu <paul@ponteilla.net>
Date: Wed, 24 Sep 2025 00:49:54 +0200
Subject: [PATCH] Add a fat12-enabled boot sector

---
 Makefile    | 35 ++++++++++++++++++++++++-------
 bootsect.S  | 29 ++++++++++++++++++++++++++
 bootsect.ld | 20 ++++++++++++++++++
 fat12boot.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 136 insertions(+), 7 deletions(-)
 create mode 100644 bootsect.S
 create mode 100644 bootsect.ld
 create mode 100644 fat12boot.c

diff --git a/Makefile b/Makefile
index b6f2a88..fdf8895 100644
--- a/Makefile
+++ b/Makefile
@@ -9,17 +9,37 @@ crc16.s: crc16.c
 crc16.bin: crc16.s crt0.c
 	ia16-elf-gcc -o crc16.bin -Os -nostdlib crc16.s crt0.c
 
-wozmon.s: wozmon.cc
-	ia16-elf-gcc -S -Os -o wozmon.s wozmon.cc
+CC = ia16-elf-gcc
+CXX = ia16-elf-gcc
+LD = ia16-elf-gcc
+CXXFLAGS = -mregparmcall -ffunction-sections -Os -flto
+CFLAGS = -mregparmcall -ffunction-sections -Os -flto
+LDFLAGS = -mregparmcall -Wl,--gc-sections -Os -nostdlib -flto
 
-wozmon.bin: wozmon.s crt0.c
-	ia16-elf-gcc -o wozmon.bin -Os -nostdlib wozmon.s crt0.c
-	truncate -s 510 wozmon.bin
-	printf "\125\252" >> wozmon.bin
+%.elf:
+	$(LD) $(LDFLAGS) $(CPPFLAGS) -o $@ $^
+
+bootsectors = fat12boot.bin wozmon.bin
+
+$(bootsectors):
+	ia16-elf-objcopy -O binary $? $@
+	truncate -s 510 $@
+	printf "\125\252" >> $@
+
+fat12boot.elf: fat12boot.o fat12.o bootsect.S
+fat12boot.elf: LDFLAGS += -T bootsect.ld
+
+fat12boot.bin: fat12boot.elf
+
+wozmon.elf: wozmon.o bootsect.S
+wozmon.elf: LDFLAGS += -T bootsect.ld
+wozmon.elf: CPPFLAGS += -DNOBPB
+
+wozmon.bin: wozmon.elf
 
 .PHONY: clean
 clean:			## Remove generated files
-	rm -rf wozmon.bin crc16.bin readfloppy.bin writefloppy.bin
+	rm -rf wozmon.bin crc16.bin readfloppy.bin writefloppy.bin fat12boot.bin
 
 .PHONY: dev-image
 dev-image:
@@ -42,6 +62,7 @@ binaries:		## Build all binaries
 	docker build --build-arg TARGET=debug.bin -o . --target=export .
 	docker build --build-arg TARGET=dosdbt.bin -o . --target=export .
 	docker build --build-arg TARGET=format.bin -o . --target=export .
+	docker build --build-arg TARGET=fat12boot.bin -o . --target=export .
 
 
 .PHONY: help
diff --git a/bootsect.S b/bootsect.S
new file mode 100644
index 0000000..233e7f2
--- /dev/null
+++ b/bootsect.S
@@ -0,0 +1,29 @@
+	.arch i8086,jumps
+	.code16
+
+.section .init
+.global _start
+_start:
+#ifndef NOBPB
+    jmp l0
+    nop
+
+.ascii "IBM  3.1"
+bpb_bytespersect:    .word 512
+bpb_sectspercluster: .byte 2
+bpb_reservedsects:   .word 1
+bbp_fats:            .byte 2
+bpb_rootentries:     .word 112
+bpb_localsectors:    .word 720
+bpb_mediadescr:      .byte 0xfd
+bpb_sectsperfat:     .word 2
+
+l0:
+#endif  // NOBPB
+    cli
+    xor %ax, %ax
+    mov %ax, %ds
+    mov %ax, %ss
+    mov $0x7c00, %sp
+    sti
+    jmp main
diff --git a/bootsect.ld b/bootsect.ld
new file mode 100644
index 0000000..d9546fd
--- /dev/null
+++ b/bootsect.ld
@@ -0,0 +1,20 @@
+SECTIONS {
+    . = 0x7c00;
+
+    .text : {
+        KEEP(*(.init))
+        *(.text*)
+    }
+
+    .rodata : {
+        *(.rodata*)
+    }
+
+    .bss (NOLOAD) : {
+        * (.bss)
+    }
+
+    .data : {
+        *(.data)
+    }
+}
diff --git a/fat12boot.c b/fat12boot.c
new file mode 100644
index 0000000..633da70
--- /dev/null
+++ b/fat12boot.c
@@ -0,0 +1,59 @@
+#include <stdint.h>
+#include <stdlib.h>
+
+#include "fat12.h"
+
+#define kPolmonAddress ((void*)0x0600)
+#define kFatAddress ((void*)0x1000)
+#define kRootDirAddress ((void*)0x1200)
+
+
+static int putchar(int c) {
+    register uint8_t khar asm ("al") = c;
+    register uint8_t func asm ("ah") = 0x0e;
+    register uint8_t page asm ("bh") = 0;
+
+    asm volatile ("int $0x10"
+            :: "r" (khar), "r" (func), "r" (page)
+            : "bp");
+
+    return c;
+}
+
+static int puts(const char* msg) {
+    while (*msg) {
+        putchar(*msg++);
+    }
+    return 0;
+}
+
+__attribute__((noreturn))
+static void die(const char* msg) {
+    puts(msg);
+    while (1) {
+    }
+    __builtin_unreachable();
+}
+
+__attribute__((noreturn))
+static void jump(void* addr) {
+    asm volatile ("ljmp $0,%0" :: "i"(addr));
+    __builtin_unreachable();
+}
+
+__attribute__((noreturn))
+static void loadpolmon() {
+    if (fat12_init(kFatAddress, kRootDirAddress)) {
+        die("fi");
+    }
+
+    if (fat12_readfile("POLMON  COM", kPolmonAddress)) {
+        die("pnf");
+    }
+
+    jump(kPolmonAddress);
+}
+
+int main() {
+    loadpolmon();
+}