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[pintos-anon] / src / threads / loader.S

diff --git a/src/threads/loader.S b/src/threads/loader.S
index d1e8309c5f194ec47ca996cf7c7c00fae1e96013..b7842d3840743c6f25ab0bceaafcbd97e74e65fd 100644 (file)
--- a/src/threads/loader.S
+++ b/src/threads/loader.S
@@ -56,27 +56,30 @@
 #define CR0_PG 0x80000000      /* Paging. */
 #define CR0_WP 0x00010000      /* Write-Protect enable in kernel mode. */
 
-# Code runs in real mode, which is a 16-bit segment.
 
 .globl start
 start:
+       
+# Code runs in real mode, which is a 16-bit segment.
        .code16
 
-# Disable interrupts.
-# String instructions go upward.
+# Disable interrupts, because we will not be prepared to handle them
+# in protected mode until much later.
+# String instructions go upward (e.g. for "rep stosl" below).
 
        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
@@ -116,12 +119,13 @@ start:
        movb $0x88, %ah
        int $0x15
        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_pages
+       movl %eax, ram_pgs
        
 #### Create temporary page directory and page table and set page
 #### directory base register.
@@ -136,7 +140,9 @@ start:
 
 # Add PDEs to point to PTEs for the first 64 MB of RAM.
 # Also add identical PDEs starting at LOADER_PHYS_BASE.
-# See [IA32-v3] section 3.7.6 for a description of the bits in %eax.
+# 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 $0x11, %ecx
@@ -148,7 +154,8 @@ start:
        loop 1b
 
 # Set up one-to-map linear to physical map for the first 64 MB of RAM.
-# See [IA32-v3] section 3.7.6 for a description of the bits in %eax.
+# See [IA32-v3a] section 3.7.6 "Page-Directory and Page-Table Entries"
+# for a description of the bits in %eax.
 
        movw $0x1100, %ax
        movw %ax, %es
@@ -167,11 +174,9 @@ start:
        
 #### Switch to protected mode.
 
-# First we turn off interrupts because we don't set up an IDT.
-
-       cli
+# Note that interrupts are still off.
 
-# 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).
 
@@ -217,6 +222,17 @@ start:
 
        movl $1, %ebx
        movl $LOADER_KERN_BASE + LOADER_PHYS_BASE, %edi
+
+# Disable interrupt delivery by IDE controller, because we will be
+# polling for data.
+# (If we don't do this, Bochs 2.2.6 will never deliver any IDE
+# interrupt to us later after we reset the interrupt controller during
+# boot, even if we also reset the IDE controller.)
+
+       movw $0x3f6, %dx
+       movb $0x02, %al
+       outb %al, %dx
+       
 read_sector:
 
 # Poll status register while controller busy.
@@ -295,10 +311,10 @@ 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
@@ -307,22 +323,27 @@ 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