.text

# sonoff is a subroutine to illustrate Intel IN, OUT instructions for
# I/O, and OR/AND logical instructions

# the parameter is 0 to turn the speaker off, 1 to turn it on

# there is a one-byte I/O port at location 0x61; the least-significant 2
# bits must be set to 11 for the speaker to be on, 00 for off; the other
# bits contain other important information and must not be changed!

# note:  in Linux, I/O can be done only in Kernel Mode

# OR dest,src does the following:  for every bit in src which is a 1,
# the corresponding bit in dest is set to 1, leaving all other bits
# intact

# AND dest,src does the following:  for every bit in src which is a 0,
# the corresponding bit in dest is set to 0, leaving all other bits
# intact

.globl sonoff
sonoff:

   cmpl 4(%esp), $0  # check parameter
   jz off
   
   # turn speaker on
   inb $0x61,%al  # `b' refers to byte-sized operand, like `l' for long;
                  # %al is AL, the lower byte of AX
   orb $3,%al  # set 2 rightmost bits to 11, leaving all others intact
   outb %al,$0x61
   ret

off:

   inb $0x61,%al
   andb $0xfc,%al  # put 00 in 2 rightmost bits, leaving all others intact
   outb %al,$0x61
   ret

# conoff is a subroutine to check whether the speaker is on, returning 0
# if off, 1 if on; again, note that we wish to keep the other bits at
# port 0x61 intact

.globl conoff
conoff:

   inb $0x61, %al
   andb $3, %al
   jnz on

   # off
   movl $0, %eax
   ret

on:

   movl $1, %eax
   ret


# count1s is a subroutine to illustrate the Intel SHR instruction; it
# counts the number of 1 bits in the parameter, returning the count in
# EAX

# SHR dest,src shifts dest to the right by src bits, filling in 0s on
# the left end where bits are vacated by the shift

# SHL does the same except that the shift is leftward, filling in 0s on
# the right

# the TEST instruction is the same as AND, but does not change the
# destination; it simply sets the Flags register according to the result
# of the AND operation (TEST is to AND as CMP is to SUB)

.globl count1s
count1s:

   pushl %ebx
   pushl %ecx

   movl $0, %eax  # initialize 1s count
   movl $32, %ebx  # initialize loop counter
   movl 12(%esp), %ecx  # get parameter

top:

   testl $1, %ecx
   jz bitnot1

   #rightmost bit of ECX is a 1
   incl %eax

bitnot1:

   shrl $1, %ecx  # `l' stands for long, as usual
   decl %ebx
   jnz top

   ret