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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