# Copyright (c) 2002, Jason Riffel - TotalEmbedded LLC.
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without 
# modification, are permitted provided that the following conditions 
# are met:
#
# Redistributions of source code must retain the above copyright 
# notice, this list of conditions and the following disclaimer. 
#
# Redistributions in binary form must reproduce the above copyright
# notice, this list of conditions and the following disclaimer in 
# the documentation and/or other materials provided with the
# distribution. 
#
# Neither the name of TotalEmbedded nor the names of its 
# contributors may be used to endorse or promote products derived 
# from this software without specific prior written permission.
# 
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 
# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 
# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS 
# FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE 
# COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, 
# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, 
# BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; 
# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER 
# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 
# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN 
# ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 
# POSSIBILITY OF SUCH DAMAGE.
#
	.text
	.align	2
	.globl start
	.ent start
start:
	.set noat
	# Use memory location 0xFF2FFFFC which lives in EJTAG dmseg as a push
	# pop stack.  The EJTAG port will handle the magic.  All we have to do
	# is write and read things ito this address in a LIFO format. This 
	# way we can use the ONLY scratch register to hold the original value
	# of R15, load R15 with our push/pop stack address, and then we can
	# push any registers we so desire at the location pointed to by R15.
	# On exit we pop out everything from the EJTAG stack and restore R15
	# from the scratch register.
	mtc0	$15, $31 
	lui	$15, 0xFF2F
	ori	$15, $15, 0xFFFC

	# Store R1 R2 R3 on the debug stack
	sw	$1,  ($15)
	sw	$2,  ($15)
	sw	$3,  ($15)

	# Load R1 with the address of the address register
	lui	$1,  0xFF2F
	ori	$1,  0xFE00
	
	# Load R2 with the address for the write
	lw	$2,  ($1)

	# Move R1 to the data register
	addiu	$1, $1, 0x04
	
	# Load R3 with the data from @R1
	lw	$3, 0($1)
	
	# Store the half word at @R2 (the address)
	sh	$3,  ($2)

	# Pop our stored R1 R2 R3 in order
	lw	$3,  ($15)
	lw	$2,  ($15)
	lw	$1,  ($15)

	# Restore R15 from the scratch register
	mfc0	$15, $31 

	# !!! MUST USE BEQ TO RETURN !!!
	# This is because an absolute jump to 0xFF200200 will not be
	# encoded properly by the listing from the .S file.  The assignment
	# of physical addresses is done by the linker which is never invoked.
	# Using of BEQ uses a relative address from the PC which is always
	# valid.  Use a compare of R0 to R0 since its always 0 its always
	# equal.  You must also insert NOP's because the assembler assumes
	# the valid usage of branch delay slots.  The assembler will insert
	# the previous instruction AFTER the branch because at runtime the 
	# processor uses delay slots (executes an instruction AFTER the branch
	# because of the pipeline).  This doesn't happen in debug mode over
	# EJTAG so we must insert NOP's to fill the slot and make sure the
	# instructions are executed in the order we want.
	nop
	beq	$0, $0, start
	
	.end	start