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##################################################################
# TITLE: Boot Up Code
# AUTHOR: Steve Rhoads (rhoadss@yahoo.com)
# DATE CREATED: 1/12/02
# FILENAME: boot.asm
# PROJECT: Plasma CPU core
# COPYRIGHT: Software placed into the public domain by the author.
#    Software 'as is' without warranty.  Author liable for nothing.
# DESCRIPTION:
#    Initializes the stack pointer and jumps to main2().
##################################################################
        .text
        .align  2
        .globl  entry
        .ent    entry
entry:
   .set noreorder

   #These eight instructions must be the first instructions.
   #convert.exe will correctly initialize $gp
   lui   $gp, 0
   ori   $gp, $gp, 0
   #convert.exe will set $4=.sbss_start $5=.bss_end
   lui   $4, 0
   ori   $4, $4, 0
   lui   $5, 0
   ori   $5, $5, 0
   lui   $sp, 0
   ori   $sp, $sp, 0xfff0     #initialize stack pointer
$BSS_CLEAR:
   sw    $0, 0($4)
   slt   $3, $4, $5
   bnez  $3, $BSS_CLEAR
   addiu $4, $4, 4

   jal   main2
   nop
$L1:
   j $L1

   #address 0x3c
interrupt_service_routine:
   #registers $26 and $27 are reserved for the OS
   ori   $26, $0, 0xffff
   ori   $27, $0, 46
   sb    $27, 0($26)         #echo out '.'
   
   #normally clear the interrupt source here

   #return and re-enable interrupts
   ori   $26, $0, 0x1
   mfc0  $27, $14      #C0_EPC=14
   jr    $27
   mtc0  $26, $12      #STATUS=1; enable interrupts
   .set reorder
   .end entry


###################################################
   .globl isr_enable
   .ent isr_enable
isr_enable:
   .set noreorder
   jr    $31
   mtc0  $4, $12            #STATUS=1; enable interrupts
   .set reorder
   .end isr_enable


###################################################
        .globl  putchar
        .ent    putchar
putchar:
   .set noreorder
   li    $5, 0xffff

   #Uncomment to make each character on a seperate line
   #The VHDL simulator buffers the lines
#   sb   $4, 0($5)
#   ori  $4, $0, '\n'

   jr    $31
   sb    $4, 0($5)
   .set reorder
   .end putchar


###################################################
        .globl  puts
        .ent    puts
puts:
   .set noreorder
   ori   $5,$0,0xffff
PUTS1:
   lb    $6, 0($4)
   beqz  $6, PUTS2
   addiu $4, $4, 1
   b     PUTS1
   sb    $6, 0($5)
PUTS2:
   jr    $31
   ori   $2, $0, 0
   .set reorder
   .end puts