CPS 104 (Ramm)
                     Fall Semester 1995
                   Homework Assignment #4

Due Monday, Oct. 2

Write an assembler subroutine designed to read in a decimal
number from the keyboard.  Call this subroutine decin and
have it work in the following way.  It should read in a
string from the keyboard using system call code 8.  It
should then go through that (null terminated) string a
character (byte) at a time until it encounters the first
valid decimal character(digit).  All non-decimal characters
up to that point should be ignored.  Once it finds decimal
characters, it should start converting these to an integer
until another non-decimal character is read in.  Then it
should return the value of this number in the $v0 register.
If the number being read in is too large for the register,
your program should go blindly ahead, keeping only the 32
least significant bits of the number entered.

To show that your program is working, call decin in a loop,
following each call to decin with a call to the integer
output system call (code 1) to show that the value was
properly read in and converted.  Have the program exit the
loop when a value of zero is returned by decin.

It is good programming practice to have subroutines save
registers on entry to the subroutine and to restore these
when the subroutine exits.  Support the convention specified
in the table on page 10 of your SPIM manual.  It is
unnecessary (and inefficient) to save registers that are not
altered in the subroutine.  In this assignment and in future
assignments, subroutines which do not follow this practice
of saving and restoring the appropriate environment will be
counted in error, unless a strong, written case is made (in
the comments, for example) for not saving register, etc.




















                     September 22, 1995