Kernel now is loaded from a partition, allowing a separate MBR bootloader if desired

Makefile

@@ -7,16 +7,29 @@ LFLAGS = -Wl,--gc-sections -Wl,--print-gc-sections -m32 -nostartfiles -nostdlib
 ifeq ($(OUTFILE),)
 OUTFILE = virtdisk.bin
 endif
+ifeq ($(PARTSTART),)
+PARTSTART = 2048
+endif
+
+PARTVBR=$(shell echo "$(PARTSTART) * (2^9)" | bc)
+PARTVBRBOOT=$(shell echo "$(PARTVBR) + 90" | bc)
+KERNSEC=$(shell echo "$(PARTSTART) + 4" | bc)
 
 .PHONY: $(OUTFILE)
 
 all: $(OUTFILE)
 
-$(OUTFILE): boot.bin kernel.bin
+$(OUTFILE): boot.bin boot_partition.bin kernel.bin
-	# Copy to boot sector, don't overwrite MBR
+	# Copy system bootloader to boot sector, don't overwrite MBR
 	dd bs=400 count=1 conv=notrunc if=boot.bin of=$@
-	# Write kernel beyond boot sector, maximum 128K (256 sectors)
+	# Ensure partition VBR contains EB 5A
-	dd bs=512 count=256 seek=1 conv=notrunc if=kernel.bin of=$@
+	echo -n -e '\xeb\x5a' | dd bs=1 seek=$(PARTVBR) count=2 conv=notrunc of=$@
+	# Copy kernel bootloader to partition VBR
+	dd bs=1 count=420 seek=$(PARTVBRBOOT) conv=notrunc if=boot_partition.bin of=$@
+	# TODO Check that disk has enough reserved sectors,
+	# currently this will overwrite the disk if too few
+	# Write kernel beyond boot sector, maximum 64K (128 sectors)
+	dd bs=512 count=128 seek=$(KERNSEC) conv=notrunc if=kernel.bin of=$@
 
 kernel.bin: out.o link.ld usermode.o
 	clang $(LFLAGS) -Wl,-M -Tlink.ld -ffreestanding -o $@ out.o usermode.o
@@ -37,8 +50,7 @@ out.o: $(objects)
 	clang $(CFLAGS) -ffunction-sections -fdata-sections -Os -o $@ $<
 
 virtdisk:
-	dd bs=1M count=32 if=/dev/zero of=virtdisk.bin
+	cp virtdisk.bin.ex virtdisk.bin
-	echo -n -e '\x55\xaa' | dd bs=1 seek=510 conv=notrunc of=virtdisk.bin
 
 clean:
 	rm -f $(objects) out.o kernel.bin boot.bin usermode.bin

boot.nasm

@@ -1,6 +1,8 @@
-secreadcnt equ 0x38
+; This boots the active partition.
-kernelreads equ 0x10000/(512*secreadcnt) ; 64K / Sector Size FIXME This underestimates when kernel size is not divisible by bytes per read
+; Relocates self to 0x7E00, loads the 
-[ORG 0x7c00]
+; first sector of active partition
+; to 0x7C00 and jumps
+[ORG 0x7C00]
 [BITS 16]
 xor ax, ax
 mov ds, ax
@@ -8,68 +10,50 @@ mov es, ax
 mov ax, 0x8000
 mov ss, ax
 mov sp, 0xFF00
+; Relocate self
+mov di, 0x7E00
+mov si, 0x7C00
+mov cx, 512
+rep movsw
+jmp 0:relocated
+; TODO Make this calculated, somehow
+[SECTION RELOC vstart=0x7E20]
+relocated:
 mov ah, 0x41 ; int 13 extensions check
 mov bx, 0x55AA
 int 0x13
-jc no_exten
+jc no_int13
-mov cx, kernelreads
+; Find active partition
-read_loop:
+xor cx, cx
+mov si, 0x7E00+0x1BE
+.find_active:
+lodsb
+bt ax, 7
+jc read
+add si, 15
+inc cl
+cmp cl, 4
+jl .find_active
+jmp err
+read:
+; Put partition start LBA in disk address packet
+add si, 7
+mov di, addr_packet_start_block
+movsw
+movsw
+; Load the first sector of the partition
 xor ax, ax
 mov ah, 0x42
 mov si, addr_packet
 int 0x13
 jc err
-add word [addr_packet_transfer_buff_seg], (secreadcnt*512)/16
+; Jump to partition boot
-add word [addr_packet_start_block], secreadcnt
+jmp 0:0x7C00
-loop read_loop
+
-entry:
+no_int13:
-;mov ax,0x2403 ; A20 BIOS Support
+mov si, no_exten_str
-;int 0x15
+mov cx, no_exten_str_end - no_exten_str
-;jb .no_a20
+jmp print
-;cmp ah,0
-;jnz .no_a20
-;mov ax,0x2401 ; A20 BIOS Activate
-;int 0x15
-;.no_a20:
-cli             ; no interrupts
-xor ax,ax
-mov ds, ax
-lgdt [gdt_desc]  ; load gdt register
-mov eax, cr0    ; set pmode bit
-or al, 1
-mov cr0, eax
-jmp 08h:Pmode
-no_exten:
-; read with CHS
-; FIXME This only reads up to sector count
-mov ah,8
-int 0x13 ; geometry
-and cx, 0x3f
-push cx
-push 0x0080 ; DH=head,DL=disk
-push 0x2000 ; buffer seg
-push 2 ; sector
-.loop:
-mov ax, [esp]
-push 0xb800
-pop es
-xor di,di
-call hexprint
-mov ax, [esp+2]
-mov es, ax
-mov ax, 0x0201
-mov cx, [esp]
-xor bx,bx
-mov dx, [esp+4]
-int 0x13
-jc err
-add word [esp+2], 0x20
-mov cx, [esp]
-inc cl
-mov [esp], cx
-cmp cl, [esp+6]
-jg entry
-jmp .loop
 err:
 mov bx, ax
 mov si, string
@@ -83,126 +67,17 @@ err_print:
 lodsb
 stosw
 loop err_print
-;add di, 2
-;mov ax, bx
-;call hexprint
 hlt_loop:
 hlt
 jmp hlt_loop
-hexprint:
-xor cx, cx
-mov bl, al
-shr al, 4
-jmp .donibble
-.nibble2:
-mov al, bl
-inc cx
-.donibble:
-and al, 0x0F
-cmp al, 0x0A
-jl .noadjust
-add al, 'A' - '0' - 10
-.noadjust:
-add al, '0'
-mov ah, 0x7
-stosw
-test cx, cx
-jz .nibble2
-ret
-
-[BITS 32]
-Pmode:
-mov eax, 0x10
-mov ds, ax
-mov es, ax
-mov fs, ax
-mov gs, ax
-mov ss, ax
-; check A20
-.a20:
-mov edi, 0x107C00
-mov esi, 0x007C00
-mov [esi], esi
-mov [edi], edi
-cmpsd
-jne .kernel
-call enable_A20
-;in al, 0x92 ; fast A20
-;test al, 2
-;jnz .fa20_end
-;or al, 2
-;and al, 0xFE
-;out 0x92, al
-;.fa20_end:
-jmp .a20
-.kernel:
-;mov dword [0xb8000], 0x07000700 | 'P' | 'M' << 16
-;mov dword [0xb8004], 0x07000700 | 'O' | 'D' << 16
-;mov dword [0xb8008], 0x07000700 | 'E' | ' ' << 16
-mov esi, 0x20000
-mov edi, 0x100000
-mov ecx, 0x10000
-rep movsb
-jmp 08h:0x100000
-enable_A20:
-        cli
-        call    a20wait
-        mov     al,0xAD
-        out     0x64,al
-        call    a20wait
-        mov     al,0xD0
-        out     0x64,al
-        call    a20wait2
-        in      al,0x60
-        push    eax
-        call    a20wait
-        mov     al,0xD1
-        out     0x64,al
-        call    a20wait
-        pop     eax
-        or      al,2
-        out     0x60,al
-        call    a20wait
-        mov     al,0xAE
-        out     0x64,al
-        call    a20wait
-        ret
-a20wait:
-        in      al,0x64
-        test    al,2
-        jnz     a20wait
-        ret
-a20wait2:
-        in      al,0x64
-        test    al,1
-        jz      a20wait2
-        ret
-
-gdt_desc:
-    dw gdt_end - gdt
-    dd gdt
-gdt:
-gdt_null: dq 0
-gdt_code: dw 0xFFFF, 0     ; bits 0-15 limit (4GB), bits 0-15 base address
-          db 0             ; bits 16-23 base address
-          db 10011010b     ; access byte
-          db 11001111b     ; bits 16-19 limit (4GB), 4 bits flags
-          db 0 ; bits 24-31 base address
-gdt_data: dw 0xFFFF, 0     ; bits 0-15 limit (4GB), bits 0-15 base address
-          db 0             ; bits 16-23 base address
-          db 10010010b     ; access byte
-          db 11001111b     ; bits 16-19 limit (4GB), 4 bits flags
-          db 0 ; bits 24-31 base address
-gdt_end:
-
 string: db 'DISK ERROR'
 string_end:
-;no_exten_str: db 'NO INT13 EXTEN'
+no_exten_str: db 'NO INT13 EXTEN'
-;no_exten_str_end:
+no_exten_str_end:
 
 addr_packet:
 db 0x10, 0x00 ; size, reserved
-dw secreadcnt ; blocks
+dw 1 ; blocks
-addr_packet_transfer_buff_off: dw 0x0000 ; transfer buffer offset
+addr_packet_transfer_buff_off: dw 0x7C00 ; transfer buffer offset
-addr_packet_transfer_buff_seg: dw 0x2000 ; transfer buffer segment
+addr_packet_transfer_buff_seg: dw 0x0000 ; transfer buffer segment
-addr_packet_start_block: dq 1 ; start block
+addr_packet_start_block: dq 0 ; start block

boot_partition.nasm

@@ -0,0 +1,136 @@
+; This boots *from a FAT partition*
+; So it starts at 0x7C00+90, the offset of
+; the boot code in a FAT VBR
+secreadcnt equ 0x38
+kernelreads equ 0x10000/(512*secreadcnt) ; 64K / Sector Size FIXME This underestimates when kernel size is not divisible by bytes per read
+[ORG 0x7c00+90]
+[BITS 16]
+xor ax, ax
+mov ds, ax
+mov es, ax
+mov ax, 0x8000
+mov ss, ax
+mov sp, 0xFF00
+; FIXME Assumes INT 13 Extension support
+mov ah, 0x42
+mov si, addr_packet
+int 0x13
+jc err
+xor cx, cx
+mov si, 0x7E00+0x1BE
+.find_active:
+lodsb
+bt ax, 7
+jc read
+add si, 15
+inc cl
+cmp cl, 4
+jl .find_active
+jmp err
+read:
+mov dh, cl ; Store active partition
+add si, 7
+mov di, addr_packet_start_block
+lodsw
+; Offset of kernel in partition
+add ax, 4
+stosw
+lodsw
+adc ax, 0
+stosw
+mov cx, kernelreads
+read_loop:
+xor ax, ax
+mov ah, 0x42
+mov si, addr_packet
+int 0x13
+jc err
+add word [addr_packet_transfer_buff_seg], (secreadcnt*512)/16
+add word [addr_packet_start_block], secreadcnt
+loop read_loop
+entry:
+cli             ; no interrupts
+xor ax,ax
+mov ds, ax
+lgdt [gdt_desc]  ; load gdt register
+mov eax, cr0    ; set pmode bit
+or al, 1
+mov cr0, eax
+jmp 08h:Pmode
+err:
+mov si, string
+mov cx, string_end - string
+print:
+push 0xb800
+pop es
+xor di, di
+mov ah, 0x7
+err_print:
+lodsb
+stosw
+loop err_print
+hlt_loop:
+hlt
+jmp hlt_loop
+
+[BITS 32]
+Pmode:
+mov eax, 0x10
+mov ds, ax
+mov es, ax
+mov fs, ax
+mov gs, ax
+mov ss, ax
+; check A20
+.a20:
+mov edi, 0x107C00
+mov esi, 0x007C00
+mov [esi], esi
+mov [edi], edi
+cmpsd
+jne .kernel
+; FIXME It's annoying that fast A20
+; will crash old systems, but it's
+; just so small...
+in al, 0x92 ; fast A20
+test al, 2
+jnz .fa20_end
+or al, 2
+and al, 0xFE
+out 0x92, al
+.fa20_end:
+jmp .a20
+.kernel:
+mov esi, 0x7E00
+mov edi, 0x100000
+mov ecx, 0x10000
+rep movsb
+xchg bx,bx
+jmp 08h:0x100000
+
+gdt_desc:
+    dw gdt_end - gdt
+    dd gdt
+gdt:
+gdt_null: dq 0
+gdt_code: dw 0xFFFF, 0     ; bits 0-15 limit (4GB), bits 0-15 base address
+          db 0             ; bits 16-23 base address
+          db 10011010b     ; access byte
+          db 11001111b     ; bits 16-19 limit (4GB), 4 bits flags
+          db 0 ; bits 24-31 base address
+gdt_data: dw 0xFFFF, 0     ; bits 0-15 limit (4GB), bits 0-15 base address
+          db 0             ; bits 16-23 base address
+          db 10010010b     ; access byte
+          db 11001111b     ; bits 16-19 limit (4GB), 4 bits flags
+          db 0 ; bits 24-31 base address
+gdt_end:
+
+string: db 'DISK ERROR'
+string_end:
+
+addr_packet:
+db 0x10, 0x00 ; size, reserved
+dw secreadcnt ; blocks
+addr_packet_transfer_buff_off: dw 0x7E00 ; transfer buffer offset
+addr_packet_transfer_buff_seg: dw 0x0000 ; transfer buffer segment
+addr_packet_start_block: dq 0 ; start block

helper.c

@@ -48,11 +48,13 @@ void SetCursorDisabled() {
     V8086Int(0x10, &regs);
 }
 
+// This should DEFINITELY be an argument
+uint8_t SystemPartition = 0;
 uint32_t OpenVol(VOLINFO *vi) {
     uint8_t *diskReadBuf = (uint8_t *)0x20000;
     uint8_t pactive, ptype;
     uint32_t pstart, psize;
-    pstart = DFS_GetPtnStart(0, diskReadBuf, 0, &pactive, &ptype, &psize);
+    pstart = DFS_GetPtnStart(0, diskReadBuf, SystemPartition, &pactive, &ptype, &psize);
     if (pstart == -1) return -1;
     return DFS_GetVolInfo(0, diskReadBuf, pstart, vi);
 }

kernel.c

@@ -101,6 +101,14 @@ void ensure_v86env() {
         *d++ = *s++;
 }
 
+uint32_t _ERRORCODE = 0;
+__attribute__((__no_caller_saved_registers__))
+uint32_t check_error_code() {
+    uint32_t v = _ERRORCODE;
+    _ERRORCODE = 0;
+    return v;
+}
+
 __attribute__((__no_caller_saved_registers__))
 __attribute__((__noreturn__))
 extern void return_prev_task();
@@ -145,6 +153,7 @@ void error_environment(uint32_t stack0, uint32_t stack1, uint32_t stack2, uint32
     // reset error screen
     for (int i = 0; i < (80*50)/2; i++)
         ((uint32_t*)error_screen)[i] = 0x0f000f00;
+    _ERRORCODE = -1;
     return_prev_task();
 }
 
@@ -181,6 +190,10 @@ Protected Only (1MB+)
 400000 - 700000 Usermode Code (3mB)
 700000 - 800000 Usermode Stack (1mB)
 */
+
+// FIXME Truly awful
+extern uint8_t SystemPartition;
+
 void DrawScreen(uint16_t *vga) {
     uint16_t *vga_text = vga;
     // clear screen
@@ -214,6 +227,7 @@ void DrawScreen(uint16_t *vga) {
     vga_text[80+42] = 0x1f00 | 'S';
     vga_text[80+43] = 0x1f00 | ' ';
     vga_text[80+44] = 0x1f00 | '-';
+    printByte(SystemPartition, &vga_text[80+50]);
 }
 
 void SetPalette() {
@@ -418,7 +432,9 @@ void SystemRun() {
     {
         VOLINFO vi;
         // TODO Check partitions beyond 0
-        while (OpenVol(&vi)) {
+        while (1) {
+            create_child(GetFreeStack(), (uintptr_t)OpenVol, 1, &vi);
+            if (!check_error_code()) break;
             vga_text = &((word*)0xb8000)[80*4 + 2];
             vga_text += printStr("Error loading file select. Ensure the disk has a valid MBR and FAT partition.", vga_text);
             vga_text = &((word*)0xb8000)[80*5 + 2];
@@ -457,8 +473,8 @@ void start() {
     vga_text = &vga_text[80];
 
     // DL *should* be preserved
-    uint8_t dl;
+    uint16_t boot_dx;
-    asm volatile("nop":"=d"(dl));
+    asm volatile("nop":"=d"(boot_dx));
 
     if (!check_cmov()) {
         char cmov_err[] = "NO CMOV";
@@ -467,7 +483,7 @@ void start() {
         for (;;) asm volatile("hlt");
     }
 
-    vga_text += printByte(dl, vga_text);
+    vga_text += printWord(boot_dx, vga_text);
     vga_text++;
 
     uint32_t o;
@@ -509,6 +525,9 @@ void start() {
     backup_ivtbios();
     InitDisk();
 
+    // DL contained disk number, DH contained active partition
+    SystemPartition = boot_dx >> 8;
+
     create_child(GetFreeStack(), (uintptr_t)SystemRun, 0);
     // If this returns, something is *very* wrong, reboot the system
     // TODO Maybe try to recover?

tests.c

@@ -122,6 +122,10 @@ void TestCHS() {
         }
     }
 }
+
+// FIXME Just horrible
+extern uint8_t SystemPartition;
+
 void TestFAT() {
     uint16_t *vga_text = (uint16_t *)0xb8000;
     uint8_t *diskReadBuf = (uint8_t *)0x22400;
@@ -131,7 +135,7 @@ void TestFAT() {
 
     uint8_t pactive, ptype;
     uint32_t pstart, psize;
-    pstart = DFS_GetPtnStart(0, diskReadBuf, 0, &pactive, &ptype, &psize);
+    pstart = DFS_GetPtnStart(0, diskReadBuf, SystemPartition, &pactive, &ptype, &psize);
     vga_text = (uint16_t *)0xb8000;
     vga_text += printStr("PartStart: ", vga_text);
     vga_text += printDword(pstart, vga_text);