#  echo2.asm
#
#  Written by:  Dan Stearns
#  For:         CPE 315 
#  Date:        11/17/02
#
#  Modifications:
#    2/28/08 -  removed loop counter; used byte instructions
#
#
#  This program echos characters forever on the SPIM console
#     using the SPIM memory-mapped I/O.
#
#  Notes:
#  1.  To run:   xspim  -noexception -mapped_io echo.asm
#

.globl __start

# bit 0 is the ready bit 
RCVR_CONTROL = 0xffff0000

	.ktext
__start:

# load Receiver control address
	la  $t0, RCVR_CONTROL 

# poll for an input character 
poll:   lbu $t4, 0($t0)		
	and $t4, $t4, 1
	beq $t4, 0, poll

# get the character 
	lbu $t5, 4($t0)		

# echo the character to console
hang:   lbu $t4, 8($t0)		
	and $t4, $t4, 1
	beq $t4, 0, hang
        sb  $t5, 0xC($t0)

# go wait for next character
        beq $0,$0,poll