[exeprefix] Add .exe prefix

An iPXE .exe image can be loaded from DOS.  Tested using bin/ipxe.exe
to load a Linux kernel and simple initramfs from within MS-DOS 6.22.
(EDD must be disabled using the "edd=off" kernel parameter, since the
loaded kernel image has already overwritten parts of DOS' INT 13
wrapper.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/Makefile.pcbios

@@ -24,6 +24,7 @@ MEDIA		+= dsk
 MEDIA		+= nbi
 MEDIA		+= hd
 MEDIA		+= raw
+MEDIA		+= exe
 
 # Padding rules
 #
@@ -31,6 +32,7 @@ PAD_rom		= $(PERL) $(PADIMG) --blksize=512 --byte=0xff $@
 PAD_mrom	= $(PAD_rom)
 PAD_dsk		= $(PERL) $(PADIMG) --blksize=512 $@
 PAD_hd		= $(PERL) $(PADIMG) --blksize=32768 $@
+PAD_exe		= $(PERL) $(PADIMG) --blksize=512 $@
 
 # Finalisation rules
 #

src/arch/i386/prefix/exeprefix.S

@@ -0,0 +1,156 @@
+/*
+ * Copyright (C) 2011 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER )
+
+/* Initial temporary stack size */
+#define EXE_STACK_SIZE 0x400
+
+/* Temporary decompression area (avoid DOS high memory area) */
+#define EXE_DECOMPRESS_ADDRESS 0x110000
+
+/* Fields within the Program Segment Prefix */
+#define PSP_CMDLINE_LEN 0x80
+#define PSP_CMDLINE_START 0x81
+
+	.text
+	.arch i386
+	.org 0
+	.code16
+	.section ".prefix", "awx", @progbits
+
+signature:
+	/* "MZ" signature */
+	.ascii	"MZ"
+
+last_block:
+	/* Number of bytes in last block that are really used */
+	.word	0
+
+blocks:
+	/* Number of 512-byte blocks */
+	.word	0
+	.section ".zinfo.fixup", "a", @progbits /* Compressor fixups */
+	.ascii	"ADDW"
+	.long	blocks
+	.long	512
+	.long	0
+	.previous
+
+num_reloc:
+	/* Number of relocation entries stored after the header */
+	.word	0
+
+header_pgh:
+	/* Number of paragraphs in the header */
+	.word	( ( _exe_start - signature ) / 16 )
+
+min_bss_pgh:
+	/* Minimum number of paragraphs of additional (BSS) memory */
+	.word	( EXE_STACK_SIZE / 16 )
+
+max_bss_pgh:
+	/* Maximum number of paragraphs of additional (BSS) memory */
+	.word	( EXE_STACK_SIZE / 16 )
+
+init_ss:
+	/* Initial stack segment (relative to start of executable) */
+	.word	-( ( _exe_start - signature ) / 16 )
+	.section ".zinfo.fixup", "a", @progbits /* Compressor fixups */
+	.ascii	"ADDW"
+	.long	init_ss
+	.long	16
+	.long	0
+	.previous
+
+init_sp:
+	/* Initial stack pointer */
+	.word	EXE_STACK_SIZE
+
+checksum:
+	/* Checksum (ignored) */
+	.word	0
+
+init_ip:
+	/* Initial instruction pointer */
+	.word	_exe_start
+
+init_cs:
+	/* Initial code segment (relative to start of executable) */
+	.word	-( ( _exe_start - signature ) / 16 )
+
+reloc_table:
+	/* Relocation table offset */
+	.word	0
+
+overlay:
+	/* Overlay number */
+	.word	0
+
+	.align 16, 0
+
+	.globl	_exe_start
+_exe_start:
+	/* Install iPXE.  Use a fixed temporary decompression area to
+	 * avoid trashing the DOS high memory area.
+	 */
+	call	alloc_basemem
+	xorl	%esi, %esi
+	movl	$EXE_DECOMPRESS_ADDRESS, %edi
+	clc
+	call	install_prealloc
+
+	/* Set up real-mode stack */
+	movw	%bx, %ss
+	movw	$_estack16, %sp
+
+	/* Jump to .text16 segment */
+	pushw	%ax
+	pushw	$1f
+	lret
+	.section ".text16", "awx", @progbits
+1:
+	/* Terminate command line with a NUL */
+	movzbw	PSP_CMDLINE_LEN, %si
+	movb	$0, PSP_CMDLINE_START(%si)
+
+	/* Calculate command line physical address */
+	xorl	%esi, %esi
+	movw	%ds, %si
+	shll	$4, %esi
+	addl	$PSP_CMDLINE_START, %esi
+
+	/* Set up %ds for access to .data16 */
+	movw	%bx, %ds
+
+	/* Record command line address */
+	movl	%esi, cmdline_phys
+
+	/* Run iPXE */
+	pushl	$main
+	pushw	%cs
+	call	prot_call
+	popl	%ecx /* discard */
+
+	/* Uninstall iPXE */
+	call	uninstall
+
+	/* Exit back to DOS.  This is very unlikely to work */
+	movw	$0x4c00, %ax
+	int	$0x21