From 71c89387fc55152bdc35c64833ff8fb6eda999a1 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Wed, 3 Feb 2021 00:27:44 +0000
Subject: [PATCH] New bootloader concept: sys-like util to generate a boot
 sector

Right now only floppies are supported
---
 Makefile                    |  25 ++---
 com/hello.asm               |  26 ++---
 fix-rom.c => host/fix-rom.c |   0
 host/sys.c                  | 202 ++++++++++++++++++++++++++++++++++++
 4 files changed, 224 insertions(+), 29 deletions(-)
 rename fix-rom.c => host/fix-rom.c (100%)
 create mode 100644 host/sys.c

diff --git a/Makefile b/Makefile
index c13a4e8..fb95607 100644
--- a/Makefile
+++ b/Makefile
@@ -23,13 +23,16 @@ ifndef DISPLAY
 QEMU_ARGS += --display curses
 endif
 
+SYS = host/sys.elf
+FIXROM = host/fix-rom.elf
+
 .PHONY: default clean qemu-floppy5 qemu-floppy3
+.PRECIOUS: $(DISTFILES)
 
 default: fd1440.img
 
 # Host utils
-utils/%: %.c
-	mkdir -p utils
+host/%.elf: host/%.c
 	$(CC) -o $@ $<
 
 # COM programs
@@ -53,26 +56,20 @@ fat6.bs: boot/fat.asm
 	$(NASM) $(NASM_ARGS) -DFAT16 -DCHS -DLARGE -o $@ $<
 
 # BIOS option roms
-%.rom: rom/%.asm utils/fix-rom
-	$(NASM) $(NASM_ARGS) -o $@ $< && utils/fix-rom $@
+%.rom: rom/%.asm $(FIXROM)
+	$(NASM) $(NASM_ARGS) -o $@ $< && $(FIXROM) $@
 
-fd%.img: fat1.bs $(DISTFILES)
-	mformat -C -i $@ -f $* -v "$(LABEL)" -B fat1.bs ::
+fd%.img: $(DISTFILES) $(SYS)
+	mformat -C -i $@ -f $* -c 2 -v "$(LABEL)" ::
 	mcopy      -i $@ $(DISTFILES) ::
-	mattrib    -i $@ +s ::kernel.bs
-
-hdimage.img: mbr.bs fd1440.img
-	cat mbr.bs fd1440.img >$@
+	$(SYS)        $@ hello.com
 
 clean:
 	rm -f *.com *.bs *.0 *.lst *.img *.bin *.rom
-	rm -rf utils
+	rm -f host/*.elf
 
 qemu-floppy3: fd1440.img $(ROMS)
 	$(QEMU) $(QEMU_ARGS) -boot a -fda fd1440.img
 
 qemu-floppy5: fd360.img $(ROMS)
 	$(QEMU) $(QEMU_ARGS) -boot a -fda fd360.img
-
-qemu-hdd: hdimage.img $(ROMS)
-	$(QEMU) $(QEMU_ARGS) -boot c -hda hdimage.img
diff --git a/com/hello.asm b/com/hello.asm
index 30c954c..3e6579b 100644
--- a/com/hello.asm
+++ b/com/hello.asm
@@ -1,19 +1,15 @@
 ; COM program displaying hello world string
 ; This is primarily for testing purposes
-org 0x0100
+	org 0x0100
 
-main:
-  mov si, string
-  mov ah, 0x0e
-  xor bx, bx
-.loop:
-  lodsb
-  test al, al
-  jz .end
-  int 0x10
-  jmp .loop
-.end:
-  ret
+main:	mov si, string
+	mov ah, 0x0e
+	xor bx, bx
+.loop:	lodsb
+	test al, al
+	jz .end
+	int 0x10
+	jmp .loop
+.end:	ret
 
-string:
-  db "Hello world!", 0x0A, 0x0D, 0
+string:	db "Hello world!", 0x0A, 0x0D, 0
diff --git a/fix-rom.c b/host/fix-rom.c
similarity index 100%
rename from fix-rom.c
rename to host/fix-rom.c
diff --git a/host/sys.c b/host/sys.c
new file mode 100644
index 0000000..d873d33
--- /dev/null
+++ b/host/sys.c
@@ -0,0 +1,202 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <stdint.h>
+#define min(a, b) (((a) < (b)) ? (a) : (b))
+// argv[1] must be floppy image
+// argv[2] must be name of boot com
+// if you expect to see memory safety, close your editor NOW!
+
+#define AH 4
+#define AX 0
+#define CX 1
+#define DH 6
+#define BX 3
+#define SP 4
+#define SI 6
+#define DI 7
+
+char* push_imm8(char* addr, int i) {
+	*(int8_t*)addr = i;
+	return(addr+1);
+}
+
+char* push_imm16(char *addr, int i) {
+	*(int16_t*)addr = i;
+	return(addr+2);
+}
+
+#define push_mov8(a, b, c) push_imm8(push_imm8(a, 0xB0+b), c);
+char* push_mov16(char* a, int b, int c) {
+	if (c) {
+		return(push_imm16(push_imm8(a, 0xB8+b), c));
+	} else {
+		// if value is 0 for 16-bit reg, xor it with itself
+		// saves a byte
+		return(push_imm16(a, 0xC031 | (b << 11) | (b << 8)));
+	}
+}
+
+char* push_jc(char* addr, char* target) {
+	int diff = target - addr;
+	return(push_imm8(push_imm8(addr, 0x72), diff-2));
+}
+
+char* push_jmp(char* addr, char* target) {
+	int diff = target - addr;
+	if (abs(diff)<0x70) {
+		return(push_imm8(push_imm8(addr, 0xEB), diff-2));
+	} else {
+		return(push_imm16(push_imm8(addr, 0xE9), diff-3));
+	}
+}
+
+int main(int argc, char** argv) {
+	// query mshowfat for first and last cluster number
+	// we expect the file to be contiguous
+	char output[64], cmd[256]= "mshowfat -i ";
+	strcat(cmd, argv[1]);
+	strcat(cmd, " '::");
+	strcat(cmd, argv[2]);
+	strcat(cmd, "'");
+	FILE* fd = popen(cmd, "r");
+	fgets(output, 64, fd);
+	int r = pclose(fd);
+	if (r > 0) {
+		perror("Unable to query mtools");
+		exit(1);
+	}
+	// parse the two numbers out of the output
+	// output is like "::kernel.bs <2-4>"
+	char* p = strchr(output, '<');
+	p++;
+	int min = strtol(p,&p,10);
+	int max = 0;
+	if (*p=='-') {
+		p++;
+		max = strtol(p,&p,10);
+		if (*p!='>') {
+			perror("Unexpected result from mshowfat");
+			exit(1);
+		}
+	} else {
+		max = min;
+	}
+	if (*p!='>') {
+		perror("Unexpected result from mshowfat");
+		exit(1);
+	} 
+	fd = fopen(argv[1], "r+");
+	if (fd < 0) {
+		perror("Cant open disk image");
+	}
+	char bootsect[512];
+	if (fread(&bootsect, 512, 1, fd) < 1) {
+		perror("Failed to read bootsector");
+		exit(1);
+	}
+	// unpack fields from bpb
+	int ss = *((uint16_t*)(bootsect+0x0B));
+	int cs = *((uint8_t*)(bootsect+0x0D));
+	int rsc = *((uint16_t*)(bootsect+0x0E));
+	int fn = *((uint8_t*)(bootsect+0x10));
+	int rde = *((uint16_t*)(bootsect+0x11));
+	int fs = *((uint16_t*)(bootsect+0x16));
+	int spt = *((uint16_t*)(bootsect+0x18));
+	int nos = *((uint16_t*)(bootsect+0x1A));
+
+	int base = rsc + (fs * fn) + ((rde * 32) / ss);
+
+	int min_s = base + (min - 2) * cs;
+	int max_s = base + (max - 2) * cs + (cs - 1);
+	int bx = 0x100;
+	int es = 0x70;
+
+	char* bs = (char*)&bootsect;
+	char* ptr = bs+0x3E;
+	char* putsfunc = ptr;
+
+	// assemble puts function
+	ptr = push_imm16(ptr, 0xAC5E);
+	ptr = push_imm16(ptr, 0xC084);
+	ptr = push_imm16(ptr, 0x0874);
+	ptr = push_imm16(ptr, 0x0EB4);
+	ptr = push_imm16(ptr, 0xDB31);
+	ptr = push_imm16(ptr, 0x10CD);
+	ptr = push_imm16(ptr, 0xF3EB);
+	ptr = push_imm16(ptr, 0xE6FF);
+
+	// assemble entry point to our main code
+	push_jmp(bs, ptr);
+
+	// assemble segment and stack setup
+	ptr = push_mov16(ptr, AX, es);
+	ptr = push_imm16(ptr, 0xD08E); // mov sp, ax
+	ptr = push_mov16(ptr, SP, 0);
+
+	// save two copies of our working segment on the stack
+	ptr = push_imm8(ptr, 0x50 + AX); // push ax
+	ptr = push_imm8(ptr, 0x50 + AX); // push ax
+
+	// assemble relocation code
+	ptr = push_mov16(ptr, AX, 0);
+	ptr = push_imm16(ptr, 0xD88E); // mov ds, ax
+	ptr = push_mov16(ptr, SI, 0x7C00);
+	ptr = push_imm16(ptr, 0xC08E); // mov es, ax
+	ptr = push_mov16(ptr, DI, (es-0x10) << 4);
+	ptr = push_mov16(ptr, CX, 0x0100);
+	ptr = push_imm16(ptr, 0xA5F3); // rep movsw (does the copy)
+
+	ptr = push_imm8(ptr, 0xEA); // jump far
+	ptr = push_imm16(ptr, ptr+4-bs+0x600);
+	ptr = push_imm16(ptr, 0);
+
+	// pop DS and ES from the stack
+	ptr = push_imm8(ptr, 0x07); // pop es
+	ptr = push_imm8(ptr, 0x1F); // pop ds
+
+	while(min_s <= max_s) {
+		int sector = (min_s % spt) + 1;
+		int tmp = min_s / spt;
+		int head = tmp % nos;
+		int track = tmp / nos;
+		int len = min(spt - sector, max_s - min_s)+1;
+
+		// memorize jump address in case of error
+		char* retry = ptr;
+		// set up registers for drive reset
+		ptr = push_imm8(push_imm8(ptr, 0xB0+AH), 0);
+		ptr = push_imm16(ptr, 0x13CD);
+		// set up registers for read call
+		ptr = push_imm16(push_imm8(ptr, 0xB8+AX), 0x0200 | len);
+		ptr = push_imm16(push_imm8(ptr, 0xB8+CX), sector | (track << 8));
+		ptr = push_imm8(push_imm8(ptr, 0xB0+DH), head);
+		ptr = push_imm16(push_imm8(ptr, 0xB8+BX), bx);
+		// set up read call
+		ptr = push_imm16(ptr, 0x13CD);
+		// jump if carry to retry addr
+		ptr = push_jc(ptr, retry);
+
+		min_s+=len;
+		bx=bx+(ss*len);
+	}
+
+	// emerg return addr
+	ptr = push_mov16(ptr, AX, 0);
+	ptr = push_imm8(ptr, 0x50 + AX);
+
+	ptr = push_imm8(ptr, 0xEA); // jmp far
+	ptr = push_imm16(ptr, 0x100);
+	ptr = push_imm16(ptr, es);
+
+	// emerg return int 0
+	ptr = bs + 0x100;
+	ptr = push_imm16(ptr, 0x18CD);
+
+	// write back bootsector
+	fseek(fd, 0, SEEK_SET);
+	if (fwrite(&bootsect, 512, 1, fd) < 1) {
+		perror("Failed to write back boot sector");
+		exit(1);
+	}
+}