[i386] Free allocated base memory on exit, if possible

Code paths that automatically allocate memory from the FBMS at 40:13
should also free it, if possible.

Freeing this memory will not be possible if either

  1. The FBMS has been modified since our allocation, or

  2. We have not been able to unhook one or more BIOS interrupt vectors.

src/arch/i386/prefix/dskprefix.S

@@ -353,6 +353,7 @@ msg1end:
 	.word 0xAA55
 
 start_runtime:
+	/* Install gPXE */
 	call	install
 
 	/* Set up real-mode stack */
@@ -368,7 +369,10 @@ start_runtime:
 	pushl	$main
 	pushw	%cs
 	call	prot_call
-	popl	%eax /* discard */
+	popl	%ecx /* discard */
+
+	/* Uninstall gPXE */
+	call	uninstall
 
 	/* Boot next device */
 	int $0x18

src/arch/i386/prefix/hdprefix.S

@@ -82,6 +82,7 @@ load_failed:
 	.byte 0x55, 0xaa
 
 start_image:
+	/* Install gPXE */
 	call	install
 
 	/* Set up real-mode stack */
@@ -97,7 +98,10 @@ start_image:
 	pushl	$main
 	pushw	%cs
 	call	prot_call
-	popl	%eax /* discard */
+	popl	%ecx /* discard */
+
+	/* Uninstall gPXE */
+	call	uninstall
 
 	/* Boot next device */
 	int $0x18

src/arch/i386/prefix/libprefix.S

@@ -537,17 +537,20 @@ install_block:
 	.code16
 	.globl	alloc_basemem
 alloc_basemem:
+	/* Preserve registers */
+	pushw	%fs
+
 	/* FBMS => %ax as segment address */
-	movw	$0x40, %ax
-	movw	%ax, %fs
+	pushw	$0x40
+	popw	%fs
 	movw	%fs:0x13, %ax
 	shlw	$6, %ax
 
-	/* .data16 segment address */
+	/* Calculate .data16 segment address */
 	subw	$_data16_memsz_pgh, %ax
 	pushw	%ax
 
-	/* .text16 segment address */
+	/* Calculate .text16 segment address */
 	subw	$_text16_memsz_pgh, %ax
 	pushw	%ax
 
@@ -555,12 +558,67 @@ alloc_basemem:
 	shrw	$6, %ax
 	movw	%ax, %fs:0x13
 
-	/* Return */
+	/* Retrieve .text16 and .data16 segment addresses */
 	popw	%ax
 	popw	%bx
+
+	/* Restore registers and return */
+	popw	%fs
 	ret
 	.size alloc_basemem, . - alloc_basemem
 
+/****************************************************************************
+ * free_basemem (real-mode near call)
+ *
+ * Free space allocated with alloc_basemem.
+ *
+ * Parameters:
+ *   %ax : .text16 segment address
+ *   %bx : .data16 segment address
+ * Returns:
+ *   %ax : 0 if successfully freed
+ * Corrupts:
+ *   none
+ ****************************************************************************
+ */
+	.section ".text16"
+	.code16
+	.globl	free_basemem
+free_basemem:
+	/* Preserve registers */
+	pushw	%fs
+
+	/* Check FBMS counter */
+	pushw	%ax
+	shrw	$6, %ax
+	pushw	$0x40
+	popw	%fs
+	cmpw	%ax, %fs:0x13
+	popw	%ax
+	jne	1f
+
+	/* Check hooked interrupt count */
+	cmpw	$0, %cs:hooked_bios_interrupts
+	jne	1f
+
+	/* OK to free memory */
+	addw	$_text16_memsz_pgh, %ax
+	addw	$_data16_memsz_pgh, %ax
+	shrw	$6, %ax
+	movw	%ax, %fs:0x13
+	xorw	%ax, %ax
+
+1:	/* Restore registers and return */
+	popw	%fs
+	ret
+	.size free_basemem, . - free_basemem
+
+	.section ".text16.data"
+	.globl	hooked_bios_interrupts
+hooked_bios_interrupts:
+	.word	0
+	.size	hooked_bios_interrupts, . - hooked_bios_interrupts
+
 /****************************************************************************
  * install (real-mode near call)
  *
@@ -709,6 +767,29 @@ prot_call_vector:
 	.size prot_call_vector, . - prot_call_vector
 #endif
 
+/****************************************************************************
+ * uninstall (real-mode near call)
+ *
+ * Uninstall all text and data segments.
+ *
+ * Parameters:
+ *   %ax  : .text16 segment address
+ *   %bx  : .data16 segment address
+ * Returns:
+ *   none
+ * Corrupts:
+ *   none
+ ****************************************************************************
+ */
+	.section ".text16"
+	.code16
+	.globl uninstall
+uninstall:
+	call	free_basemem
+	ret
+	.size uninstall, . - uninstall
+
+
 
 	/* File split information for the compressor */
 #if COMPRESS

src/arch/i386/prefix/lkrnprefix.S

@@ -189,6 +189,7 @@ run_gpxe:
 	movw	%ax, %ss
 	movw	$0x7c00, %sp
 
+	/* Install gPXE */
 	call	install
 
 	/* Set up real-mode stack */
@@ -204,7 +205,10 @@ run_gpxe:
 	pushl	$main
 	pushw	%cs
 	call	prot_call
-	popl	%eax /* discard */
+	popl	%ecx /* discard */
+
+	/* Uninstall gPXE */
+	call	uninstall
 
 	/* Boot next device */
 	int $0x18

src/arch/i386/prefix/nbiprefix.S

@@ -52,7 +52,7 @@ memlen:	.long	_filesz - 512
  *****************************************************************************
  */
 entry:
-	/* Install low and high memory regions */
+	/* Install gPXE */
 	call	install
 
 	/* Jump to .text16 segment */
@@ -64,7 +64,10 @@ entry:
 	pushl	$main
 	pushw	%cs
 	call	prot_call
-	popl	%eax /* discard */
+	popl	%ecx /* discard */
+
+	/* Uninstall gPXE */
+	call	uninstall
 
 	/* Reboot system */
 	int $0x19

src/arch/i386/prefix/pxeprefix.S

@@ -318,7 +318,7 @@ print_free_basemem:
  *****************************************************************************
  */	
 finished:
-	jmp	run_etherboot
+	jmp	run_gpxe
 
 /*****************************************************************************
  * Subroutine: print segment:offset address
@@ -554,11 +554,11 @@ flags:			.word UNDI_FL_STARTED
 	.equ undi_device_size, ( . - undi_device )
 
 /*****************************************************************************
- * Run Etherboot main code
+ * Run gPXE main code
  *****************************************************************************
  */	
-run_etherboot:
-	/* Install Etherboot */
+run_gpxe:
+	/* Install gPXE */
 	call	install
 
 	/* Set up real-mode stack */
@@ -585,7 +585,10 @@ run_etherboot:
 	pushl	$main
 	pushw	%cs
 	call	prot_call
-	popl	%eax /* discard */
+	popl	%ecx /* discard */
+
+	/* Uninstall gPXE */
+	call	uninstall
 
 	/* Boot next device */
 	int	$0x18

src/arch/i386/prefix/romprefix.S

@@ -591,8 +591,11 @@ exec:	/* Set %ds = %cs */
 	pushl	$main
 	pushw	%cs
 	call	prot_call
-	/* No need to clean up stack; we are about to reload %ss:sp */
-	
+	popl	%ecx /* discard */
+
+	/* Uninstall gPXE */
+	call	uninstall
+
 	/* Restore BIOS stack */
 	movw	%dx, %ss
 	movw	%bp, %sp