From 86ef52d03267817a0442f2aa340860871ddb9e63 Mon Sep 17 00:00:00 2001
From: Ben Pfaff <blp@cs.stanford.edu>
Date: Sat, 28 Aug 2004 07:08:54 +0000
Subject: [PATCH] Work on loader to prepare for passing in a command line.

---
 src/Makefile.inc     |  7 +++---
 src/pad              | 20 +++++++++++++++++
 src/threads/init.c   |  2 +-
 src/threads/loader.S | 53 ++++++++++++++++++++++++++++++--------------
 4 files changed, 60 insertions(+), 22 deletions(-)
 create mode 100755 src/pad

diff --git a/src/Makefile.inc b/src/Makefile.inc
index 6ed310e..6ffa350 100644
--- a/src/Makefile.inc
+++ b/src/Makefile.inc
@@ -54,16 +54,15 @@ OBJECTS = $(patsubst %.c,%.o,$(patsubst %.S,%.o,$(SOURCES)))
 all: diskimage.bin
 
 intr-stubs.S: $(TOP_SRCDIR)/threads/intr-stubs.pl
-	$< > $@.tmp && mv $@.tmp $@
+	$< > $@
 
 kernel.o: $(OBJECTS)
 	ld -T $(TOP_SRCDIR)/threads/kernel.lds -o $@ $^ \
 		`$(CC) -print-libgcc-file-name`
 
 kernel.bin: kernel.o
-	objcopy -O binary -R .note -R .comment -S $< $@.data
-	perl -e 'print "\0" x (4096 - (-s "$@.data") % 4096)' > $@.pad
-	cat $@.data $@.pad > $@.tmp && mv $@.tmp $@
+	objcopy -O binary -R .note -R .comment -S $< $@.tmp
+	$(TOP_SRCDIR)/pad 4096 < $@.tmp > $@
 
 loader.bin: loader.S kernel.bin
 	gcc -c $< -DKERNEL_LOAD_PAGES=`perl -e 'print +(-s "kernel.bin") / 4096;'`
diff --git a/src/pad b/src/pad
new file mode 100755
index 0000000..82f35d3
--- /dev/null
+++ b/src/pad
@@ -0,0 +1,20 @@
+#! /usr/bin/perl
+
+if (@ARGV != 1) {
+    print STDERR "pad, for padding a file out to an even block size\n";
+    print STDERR "usage: pad BLOCKSIZE < INPUT > OUTPUT\n\n";
+    print STDERR "pad copies its input to its output, then writes\n";
+    print STDERR "zeroes to its output as necessary to make the\n";
+    print STDERR "output size an even multiple of BLOCKSIZE bytes\n";
+    exit 1;
+}
+
+$blocksize = $ARGV[0];
+$input_size = 0;
+while (read (STDIN, $input, 4096)) {
+    $input_size += length ($input);
+    print $input;
+}
+
+$odd_bytes = $input_size % $blocksize;
+print "\0" x ($blocksize - $odd_bytes) if $odd_bytes != 0;
diff --git a/src/threads/init.c b/src/threads/init.c
index f247612..6da714e 100644
--- a/src/threads/init.c
+++ b/src/threads/init.c
@@ -50,7 +50,7 @@ main (void)
   /* Calculate how much RAM the kernel uses, and find out from
      the bootloader how much RAM this machine has. */
   kernel_pages = (&_end - &_text + 4095) / 4096;
-  ram_pages = *(uint32_t *) (0x7e00 - 8);
+  ram_pages = *(uint32_t *) (0x7e00 - 6);
 
   printk ("Initializing nachos-x86, %d kB RAM detected.\n",
           ram_pages * 4);
diff --git a/src/threads/loader.S b/src/threads/loader.S
index c60e8dd..fd8285a 100644
--- a/src/threads/loader.S
+++ b/src/threads/loader.S
@@ -1,14 +1,15 @@
 #include "mmu.h"
 	
-###################################################################################
-# ENTRY POINT	
-#   This code should be stored in the first sector of the hard disk.  When the  
-#   BIOS runs, it loads this code at physical address 0x7c00 - 0x7e00 (512 bytes).
-#   Then jumps to the beginning of it, in real-mode (BIOS runs in real mode).  
-#	
+##############################################################################
+# Kernel loader.
+#
+# This code should be stored in the first sector of the hard disk.  When the  
+# BIOS runs, it loads this code at physical address 0x7c00-0x7e00 (512 bytes).
+# Then it jumps to the beginning of it, in real mode.  
 # This code switches into protected mode (32-bit mode) so that all of
-# memory can accessed, then calls into C.
-###################################################################################
+# memory can accessed, loads the kernel into memory, and jumps to the
+# first byte of the kernel, where start.S is linked.
+##############################################################################
 	
 .globl start				# Entry point	
 start:	.code16				# This runs in real mode
@@ -40,12 +41,12 @@ start:	.code16				# This runs in real mode
 #### 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.
 	
-	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
+	movb $0x88,%ah
+	int $0x15
+	jc panic		# Carry flag set on error
+2:	addl $1024,%eax		# Total kB
+	shrl $2,%eax		# Total 4 kB pages
+	movl %eax, ram_pages
 	
 #### switch from real to protected mode	
 ####     The segments in GDT allow all of physical memory to be accessed.
@@ -178,11 +179,29 @@ gdtdesc:
 	.word	0x17			# sizeof (gdt) - 1
 	.long	gdt			# address gdt
 
-##### Arrive here on error
-panic:	jmp panic
+##### To panic, we print panicmsg (with help from the BIOS) and spin.
+panic:  .code16			# We only panic in real mode.
+	movw $panicmsg, %si
+	movb $0xe, %ah
+	xorb %bh, %bh
+1:	lodsb
+	test %al, %al
+2:	jz 2b			# Spin.
+	int $0x10
+	jmp 1b
+
+panicmsg:
+	.ascii "Loader panic!\r\n"
+	.byte 0
+
+##### Command-line arguments inserted by another utility.
+##### The loader doesn't use these, but we note their
+##### location here for easy reference.
+	.org 0x200 - 0x80
+cmd_line:	
 
 ##### Memory size in 4 kB pages.
-	.org 0x200 - 8
+	.org 0x200 - 6
 ram_pages:
 	.long 0
 
-- 
2.30.2