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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