X-Git-Url: http://git.mutantstargoat.com/user/nuclear/?p=bootcensus;a=blobdiff_plain;f=src%2Fstartup.s;fp=src%2Fstartup.s;h=01090cd69c3fb15e81052074a423d24f5a4a092d;hp=ef3cd38b443a03e5f435f420cd841d987ac70012;hb=81c11bdd80190ec319a82b0402173cfb65fcbf72;hpb=7dcd5071e600f8cf48174d1fddb3dba57ec9476d

diff --git a/src/startup.s b/src/startup.s
index ef3cd38..01090cd 100644
--- a/src/startup.s
+++ b/src/startup.s
@@ -1,5 +1,5 @@
 # pcboot - bootable PC demo/game kernel
-# Copyright (C) 2018  John Tsiombikas <nuclear@member.fsf.org>
+# Copyright (C) 2018-2019  John Tsiombikas <nuclear@member.fsf.org>
 # 
 # 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
@@ -23,9 +23,13 @@
 	.extern wait_vsync
 	.extern kb_getkey
 
+	.equ STACKTOP,0x80000
+
 	# move the stack to the top of the conventional memory
 	cli
-	movl $0x80000, %esp
+	movl $STACKTOP, %esp
+	# push a 0 ret-addr to terminate gdb backtraces
+	pushl $0
 
 	# zero the BSS section
 	xor %eax, %eax
@@ -43,6 +47,34 @@ skip_bss_zero:
 	hlt
 	jmp 0b
 
+	# this is called once after memory init, to move the protected mode
+	# stack to the top of usable memory, to avoid interference from 16bit
+	# programs (as much as possible)
+	.global move_stack
+move_stack:
+	# calculate the currently used lowest address of the stack (rounded
+	# down to 4-byte alignment), to see where to start copying
+	mov %esp, %esi
+	and $0xfffffffc, %esi
+	# calculate the size we need to copy
+	mov $STACKTOP, %ecx
+	sub %esi, %ecx
+	# load the destination address to edi
+	mov 4(%esp), %edi
+	sub %ecx, %edi
+	# size in longwords
+	shr $2, %ecx
+
+	# change esp to the new stack
+	mov $STACKTOP, %ecx
+	sub %esp, %ecx
+	mov 4(%esp), %eax
+	mov %eax, %esp
+	sub %ecx, %esp
+
+	rep movsd
+	ret
+
 	.global logohack
 logohack:
 	pusha