X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=src%2Fthreads%2Floader.S;h=2d76763cffc3dcdc2aa339404bfb674360e22ae1;hb=8abbb333aea445641d967befd3ca477502ea770b;hp=42f489979dff15704d4cd3efa023c9954f03913b;hpb=b0a700d18f0a0a8c87e1a4fff3a2108e0edb0fbc;p=pintos-anon

diff --git a/src/threads/loader.S b/src/threads/loader.S
index 42f4899..2d76763 100644
--- a/src/threads/loader.S
+++ b/src/threads/loader.S
@@ -39,7 +39,6 @@
  */
 
 #include "threads/loader.h"
-/*#include "threads/mmu.h"*/
 	
 #### Kernel loader.
 
@@ -51,7 +50,7 @@
 #### memory, and jumps to the first byte of the kernel, where start.S
 #### is linked.
 	
-/* Flags in control register 0 */
+/* Flags in control register 0. */
 #define CR0_PE 0x00000001      /* Protection Enable. */
 #define CR0_EM 0x00000004      /* (Floating-point) Emulation. */
 #define CR0_PG 0x80000000      /* Paging. */
@@ -69,15 +68,16 @@ start:
 	cli
 	cld
 
-# Set up data segments and stack.
+# Set up data segments.
 
 	subw %ax, %ax
 	movw %ax, %es
 	movw %ax, %ds
 
+# Set up stack segment.
 # Stack grows downward starting from us.
-# We don't ever use the stack so this is strictly speaking
-# unnecessary.
+# We don't ever use the stack, but we call into the BIOS,
+# which might.
 
 	movw %ax, %ss
 	movw $0x7c00, %sp
@@ -111,20 +111,24 @@ start:
 #### Get memory size, via interrupt 15h function 88h.  Returns CF
 #### clear if successful, with AX = (kB of physical memory) - 1024.
 #### This only works for memory sizes <= 65 MB, which should be fine
-#### for our purposes.
+#### for our purposes.  We cap memory at 64 MB because that's all we
+#### prepare page tables for, below.
 	
 	movb $0x88, %ah
 	int $0x15
-	jc panic		# Carry flag set on error
-	addl $1024, %eax	# Total kB
-	shrl $2, %eax		# Total 4 kB pages
-	movl %eax, ram_pages
+	jc panic
+	cli			# BIOS might have enabled interrupts
+	addl $1024, %eax	# Total kB memory
+	cmp $0x10000, %eax	# Cap at 64 MB
+	jbe 1f
+	mov $0x10000, %eax
+1:	shrl $2, %eax		# Total 4 kB pages
+	movl %eax, ram_pgs
 	
 #### Create temporary page directory and page table and set page
 #### directory base register.
 
 # Create page directory at 64 kB and fill with zeroes.
-
 	mov $0x1000, %ax
 	mov %ax, %es
 	subl %eax, %eax
@@ -132,19 +136,32 @@ start:
 	movl $0x400, %ecx
 	rep stosl
 
-# Set PDEs for 0 and LOADER_PHYS_BASE to point to the page table.
-# See comments near the PG_* macros in paging.h for a description of
-# the values stored here.
+# Add PDEs to point to PTEs for the first 64 MB of RAM.
+# Also add identical PDEs starting at LOADER_PHYS_BASE.
+# See [IA32-v3a] section 3.7.6 "Page-Directory and Page-Table Entries"
+# for a description of the bits in %eax.
+	
 
 	movl $0x11007, %eax
-	movl %eax, %es:0
-	movl %eax, %es:LOADER_PHYS_BASE >> 20
+	movl $0x11, %ecx
+	subl %edi, %edi
+1:	movl %eax, %es:(%di)
+	movl %eax, %es:LOADER_PHYS_BASE >> 20(%di)
+	addw $4, %di
+	addl $0x1000, %eax
+	loop 1b
 
-# Initialize page table.
+# Set up one-to-map linear to physical map for the first 64 MB of RAM.
+# See [IA32-v3a] section 3.7.6 "Page-Directory and Page-Table Entries"
+# for a description of the bits in %eax.
 
-	movl $7, %eax
-	movl $0x400, %ecx
-1:	stosl
+	movw $0x1100, %ax
+	movw %ax, %es
+	movl $0x7, %eax
+	movl $0x4000, %ecx
+	subl %edi, %edi
+1:	movl %eax, %es:(%di)
+	addw $4, %di
 	addl $0x1000, %eax
 	loop 1b
 
@@ -155,11 +172,9 @@ start:
 	
 #### Switch to protected mode.
 
-# First we turn off interrupts because we don't set up an IDT.
+# Note that interrupts are still off.
 
-	cli
-
-# Then we point the GDTR to our GDT.  Protected mode requires a GDT.
+# Point the GDTR to our GDT.  Protected mode requires a GDT.
 # We need a data32 prefix to ensure that all 32 bits of the GDT
 # descriptor are loaded (default is to load only 24 bits).
 
@@ -198,7 +213,7 @@ start:
 	movw %ax, %fs		
 	movw %ax, %gs		
 	movw %ax, %ss
-	movl $LOADER_PHYS_BASE + 0x20000, %esp
+	movl $LOADER_PHYS_BASE + 0x30000, %esp
 
 #### Load kernel starting at physical address LOADER_KERN_BASE by
 #### frobbing the IDE controller directly.
@@ -283,10 +298,10 @@ gdtdesc:
 	.long	gdt + LOADER_PHYS_BASE	# address gdt
 
 #### Fatal error.
-#### Print panicmsg (with help from the BIOS) and spin.
+#### Print panic_message (with help from the BIOS) and spin.
 
 panic:  .code16			# We only panic in real mode.
-	movw $panicmsg, %si
+	movw $panic_message, %si
 	movb $0xe, %ah
 	subb %bh, %bh
 1:	lodsb
@@ -295,22 +310,27 @@ panic:  .code16			# We only panic in real mode.
 	int $0x10
 	jmp 1b
 
-panicmsg:
-	.ascii "Loader panic!\r\n"
+panic_message:
+	.ascii "Panic!"
 	.byte 0
 
-#### Memory size in 4 kB pages.
-	.org LOADER_RAM_PAGES - LOADER_BASE
-ram_pages:
+#### Physical memory size in 4 kB pages.
+#### This is initialized by the loader and read by the kernel.
+	.org LOADER_RAM_PGS - LOADER_BASE
+ram_pgs:
 	.long 0
 
-#### Command-line arguments inserted by another utility.
-#### The loader doesn't use these, but we note their
-#### location here for easy reference.
-	.org LOADER_CMD_LINE - LOADER_BASE
-cmd_line:
+#### Command-line arguments and their count.
+#### This is written by the `pintos' utility and read by the kernel.
+#### The loader itself does not do anything with the command line.
+	.org LOADER_ARG_CNT - LOADER_BASE
+arg_cnt:
+	.long 0
+	.org LOADER_ARGS - LOADER_BASE
+args:
 	.fill 0x80, 1, 0
 
-#### Boot-sector signature for BIOS inspection.
-	.org LOADER_BIOS_SIG - LOADER_BASE
+#### Boot-sector signature.
+#### The BIOS checks that this is set properly.
+	.org LOADER_SIG - LOADER_BASE
 	.word 0xaa55