import

Makefile

@@ -0,0 +1,30 @@
+OBJS = main.o console.o string.o kalloc.o proc.o trapasm.o
+
+CC = i386-jos-elf-gcc
+LD = i386-jos-elf-ld
+OBJCOPY = i386-jos-elf-objcopy
+OBJDUMP = i386-jos-elf-objdump
+
+xv6.img : bootblock kernel
+	dd if=/dev/zero of=xv6.img count=10000
+	dd if=bootblock of=xv6.img conv=notrunc
+	dd if=kernel of=xv6.img seek=1 conv=notrunc
+
+bootblock : bootasm.S bootmain.c
+	$(CC) -O -nostdinc -I. -c bootmain.c
+	$(CC) -nostdinc -I. -c bootasm.S
+	$(LD) -N -e start -Ttext 0x7C00 -o bootblock.o bootasm.o bootmain.o
+	$(OBJDUMP) -S bootblock.o > bootblock.asm
+	$(OBJCOPY) -S -O binary bootblock.o bootblock
+	./sign.pl bootblock
+
+kernel : $(OBJS)
+	$(LD) -Ttext 0x100000 -e main -o kernel $(OBJS)
+	$(OBJDUMP) -S kernel > kernel.asm
+
+%.o: %.c
+	$(CC) -nostdinc -I. -O -c -o $@ $<
+
+clean : 
+	rm -f bootmain.o bootasm.o bootblock.o bootblock
+	rm -f kernel main.o kernel.asm xv6.img

Notes

@@ -0,0 +1,67 @@
+bootmain.c doesn't work right if the ELF sections aren't
+sector-aligned. so you can't use ld -N. and the sections may also need
+to be non-zero length, only really matters for tiny "kernels".
+
+kernel loaded at 1 megabyte. stack same place that bootasm.S left it.
+
+kinit() should find real mem size
+  and rescue useable memory below 1 meg
+
+no paging, no use of page table hardware, just segments
+
+no user area: no magic kernel stack mapping
+  so no copying of kernel stack during fork
+  though there is a kernel stack page for each process
+
+no kernel malloc(), just kalloc() for user core
+
+user pointers aren't valid in the kernel
+
+setting up first process
+  we do want a process zero, as template
+    but not runnable
+  just set up return-from-trap frame on new kernel stack
+  fake user program that calls exec
+
+map text read-only?
+shared text?
+
+what's on the stack during a trap or sys call?
+  PUSHA before scheduler switch? for callee-saved registers.
+  segment contents?
+  what does iret need to get out of the kernel?
+  how does INT know what kernel stack to use?
+
+are interrupts turned on in the kernel? probably.
+
+per-cpu curproc
+one tss per process, or one per cpu?
+one segment array per cpu, or per process?
+
+pass curproc explicitly, or implicit from cpu #?
+  e.g. argument to newproc()?
+
+test stack expansion
+test running out of memory, process slots
+
+we can't really use a separate stack segment, since stack addresses
+need to work correctly as ordinary pointers. the same may be true of
+data vs text. how can we have a gap between data and stack, so that
+both can grow, without committing 4GB of physical memory? does this
+mean we need paging?
+
+what's the simplest way to add the paging we need?
+  one page table, re-write it each time we leave the kernel?
+  page table per process?
+  probably need to use 0-0xffffffff segments, so that
+    both data and stack pointers always work
+  so is it now worth it to make a process's phys mem contiguous?
+  or could use segment limits and 4 meg pages?
+    but limits would prevent using stack pointers as data pointers
+  how to write-protect text? not important?
+
+perhaps have fixed-size stack, put it in the data segment?
+
+oops, if kernel stack is in contiguous user phys mem, then moving
+users' memory (e.g. to expand it) will wreck any pointers into the
+kernel stack.

bootasm.S

@@ -0,0 +1,109 @@
+#define SEG_NULL						\
+	.word 0, 0;						\
+	.byte 0, 0, 0, 0
+#define SEG(type,base,lim)					\
+	.word (((lim) >> 12) & 0xffff), ((base) & 0xffff);	\
+	.byte (((base) >> 16) & 0xff), (0x90 | (type)),		\
+		(0xC0 | (((lim) >> 28) & 0xf)), (((base) >> 24) & 0xff)
+
+#define STA_X		0x8	    // Executable segment
+#define STA_E		0x4	    // Expand down (non-executable segments)
+#define STA_C		0x4	    // Conforming code segment (executable only)
+#define STA_W		0x2	    // Writeable (non-executable segments)
+#define STA_R		0x2	    // Readable (executable segments)
+#define STA_A		0x1	    // Accessed
+
+.set PROT_MODE_CSEG,0x8		# code segment selector
+.set PROT_MODE_DSEG,0x10        # data segment selector
+.set CR0_PE_ON,0x1		# protected mode enable flag
+
+###################################################################################
+# ENTRY POINT	
+#   This code should be stored in the first sector of the hard disk.
+#   After the BIOS initializes the hardware on startup or system reset,
+#   it loads this code at physical address 0x7c00 - 0x7d00 (512 bytes).
+#   Then the BIOS jumps to the beginning of it, address 0x7c00,
+#   while running in 16-bit real-mode (8086 compatibility mode).
+#   The Code Segment register (CS) is initially zero on entry.
+#	
+# This code switches into 32-bit protected mode so that all of
+# memory can accessed, then calls into C.
+###################################################################################
+
+.globl start					# Entry point	
+start:		.code16				# This runs in real mode
+		cli				# Disable interrupts
+		cld				# String operations increment
+
+		# Set up the important data segment registers (DS, ES, SS).
+		xorw	%ax,%ax			# Segment number zero
+		movw	%ax,%ds			# -> Data Segment
+		movw	%ax,%es			# -> Extra Segment
+		movw	%ax,%ss			# -> Stack Segment
+
+		# Set up the stack pointer, growing downward from 0x7c00.
+		movw	$start,%sp         	# Stack Pointer
+
+#### Enable A20:
+####   For fascinating historical reasons (related to the fact that
+####   the earliest 8086-based PCs could only address 1MB of physical memory
+####   and subsequent 80286-based PCs wanted to retain maximum compatibility),
+####   physical address line 20 is tied to low when the machine boots.
+####   Obviously this a bit of a drag for us, especially when trying to
+####   address memory above 1MB.  This code undoes this.
+
+seta20.1:	inb	$0x64,%al		# Get status
+		testb	$0x2,%al		# Busy?
+		jnz	seta20.1		# Yes
+		movb	$0xd1,%al		# Command: Write
+		outb	%al,$0x64		#  output port
+seta20.2:	inb	$0x64,%al		# Get status
+		testb	$0x2,%al		# Busy?
+		jnz	seta20.2		# Yes
+		movb	$0xdf,%al		# Enable
+		outb	%al,$0x60		#  A20
+
+#### Switch from real to protected mode	
+####     The descriptors in our GDT allow all physical memory to be accessed.
+####     Furthermore, the descriptors have base addresses of 0, so that the
+####     segment translation is a NOP, ie. virtual addresses are identical to
+####     their physical addresses.  With this setup, immediately after
+####	 enabling protected mode it will still appear to this code
+####	 that it is running directly on physical memory with no translation.
+####	 This initial NOP-translation setup is required by the processor
+####	 to ensure that the transition to protected mode occurs smoothly.
+
+real_to_prot:	cli				# Mandatory since we dont set up an IDT
+		lgdt	gdtdesc			# load GDT -- mandatory in protected mode
+		movl	%cr0, %eax		# turn on protected mode
+		orl	$CR0_PE_ON, %eax	# 
+		movl	%eax, %cr0		# 
+	        ### CPU magic: jump to relocation, flush prefetch queue, and reload %cs
+		### Has the effect of just jmp to the next instruction, but simultaneous
+		### loads CS with $PROT_MODE_CSEG.
+		ljmp	$PROT_MODE_CSEG, $protcseg
+
+#### we are in 32-bit protected mode (hence the .code32)
+.code32
+protcseg:	
+		# Set up the protected-mode data segment registers
+		movw	$PROT_MODE_DSEG, %ax	# Our data segment selector
+		movw	%ax, %ds		# -> DS: Data Segment
+		movw	%ax, %es		# -> ES: Extra Segment
+		movw	%ax, %fs		# -> FS
+		movw	%ax, %gs		# -> GS
+		movw	%ax, %ss		# -> SS: Stack Segment
+
+		call cmain			# finish the boot load from C.
+						# cmain() should not return
+spin:		jmp spin			# ..but in case it does, spin
+
+.p2align 2					# force 4 byte alignment
+gdt:
+	SEG_NULL				# null seg
+	SEG(STA_X|STA_R, 0x0, 0xffffffff)	# code seg
+	SEG(STA_W, 0x0, 0xffffffff)	        # data seg
+
+gdtdesc:
+	.word	0x17			# sizeof(gdt) - 1
+	.long	gdt			# address gdt

bootmain.c

@@ -0,0 +1,121 @@
+#include <types.h>
+#include <elf.h>
+#include <x86.h>
+
+/**********************************************************************
+ * This a dirt simple boot loader, whose sole job is to boot
+ * an elf kernel image from the first IDE hard disk.
+ *
+ * DISK LAYOUT
+ *  * This program(boot.S and main.c) is the bootloader.  It should
+ *    be stored in the first sector of the disk.
+ * 
+ *  * The 2nd sector onward holds the kernel image.
+ *	
+ *  * The kernel image must be in ELF format.
+ *
+ * BOOT UP STEPS	
+ *  * when the CPU boots it loads the BIOS into memory and executes it
+ *
+ *  * the BIOS intializes devices, sets of the interrupt routines, and
+ *    reads the first sector of the boot device(e.g., hard-drive) 
+ *    into memory and jumps to it.
+ *
+ *  * Assuming this boot loader is stored in the first sector of the
+ *    hard-drive, this code takes over...
+ *
+ *  * control starts in bootloader.S -- which sets up protected mode,
+ *    and a stack so C code then run, then calls cmain()
+ *
+ *  * cmain() in this file takes over, reads in the kernel and jumps to it.
+ **********************************************************************/
+
+#define SECTSIZE	512
+#define ELFHDR		((struct Elf *) 0x10000) // scratch space
+
+void readsect(void*, uint32_t);
+void readseg(uint32_t, uint32_t, uint32_t);
+
+void
+cmain(void)
+{
+	struct Proghdr *ph, *eph;
+
+	// read 1st page off disk
+	readseg((uint32_t) ELFHDR, SECTSIZE*8, 0);
+
+	// is this a valid ELF?
+	if (ELFHDR->e_magic != ELF_MAGIC)
+		goto bad;
+
+	// load each program segment (ignores ph flags)
+	ph = (struct Proghdr *) ((uint8_t *) ELFHDR + ELFHDR->e_phoff);
+	eph = ph + ELFHDR->e_phnum;
+	for (; ph < eph; ph++)
+		readseg(ph->p_va, ph->p_memsz, ph->p_offset);
+
+	// call the entry point from the ELF header
+	// note: does not return!
+	((void (*)(void)) (ELFHDR->e_entry & 0xFFFFFF))();
+
+bad:
+	outw(0x8A00, 0x8A00);
+	outw(0x8A00, 0x8E00);
+	while (1)
+		/* do nothing */;
+}
+
+// Read 'count' bytes at 'offset' from kernel into virtual address 'va'.
+// Might copy more than asked
+void
+readseg(uint32_t va, uint32_t count, uint32_t offset)
+{
+	uint32_t end_va;
+
+	va &= 0xFFFFFF;
+	end_va = va + count;
+
+	// round down to sector boundary
+	va &= ~(SECTSIZE - 1);
+
+	// translate from bytes to sectors, and kernel starts at sector 1
+	offset = (offset / SECTSIZE) + 1;
+
+	// If this is too slow, we could read lots of sectors at a time.
+	// We'd write more to memory than asked, but it doesn't matter --
+	// we load in increasing order.
+	while (va < end_va) {
+		readsect((uint8_t*) va, offset);
+		va += SECTSIZE;
+		offset++;
+	}
+}
+
+void
+waitdisk(void)
+{
+	// wait for disk reaady
+	while ((inb(0x1F7) & 0xC0) != 0x40)
+		/* do nothing */;
+}
+
+void
+readsect(void *dst, uint32_t offset)
+{
+	// wait for disk to be ready
+	waitdisk();
+
+	outb(0x1F2, 1);		// count = 1
+	outb(0x1F3, offset);
+	outb(0x1F4, offset >> 8);
+	outb(0x1F5, offset >> 16);
+	outb(0x1F6, (offset >> 24) | 0xE0);
+	outb(0x1F7, 0x20);	// cmd 0x20 - read sectors
+
+	// wait for disk to be ready
+	waitdisk();
+
+	// read a sector
+	insl(0x1F0, dst, SECTSIZE/4);
+}
+