CS 366 - First SPIM Programming Assignment

Due: Monday, February 10, 2003 at 11:59 pm

For this assignment, we will decode three different values from a integer value received from input. The decoding is done based on the following information:

The first 4 bits of the original value will be used to store a integer value in the range from 0 to 15. The "first four bits" are the four least significant bits at position 3, 2, 1 and 0. If these bits are all zeros, the decoded value will be zero. If these bits are all ones, the decoded value will be fifteen. Treat these bits as a 4-bit unsigned integer value.
The next 6 bits (position 9 through 4) of the original value will be used to store a signed integered value in the range from -32 to +31. Treat these bits as a signed 6-bit integer value using two's complement representation. If the bits are all zeros, the decoded value will be zero. If these bits are all ones, the decoded value will be -1. If these bits are "011111", the decoded value will be +31. If these bits are "100000", the decoded value will be -32.
The next 7 bits (position 16 through 10) of the original value will be used to store an ASCII character value in the numeric range from 0 to 127.
The remaining bit are not used with the decoding, so the values of these bits do not matter.

Your program is to prompt the user to enter in the input. Your program is to continue prompting, reading and decoding integer values until a negative value is entered. When an negative value is entered, print some appropriate message and quit the program. For a non-negative integer input, print a message clearly showing the three decoded values. When printing the ASCII character value, print out the actual character when possible. If the character is a non-printable character, print out a message stating the character is non-printable and print out the numeric value of the ASCII character.

The programs will be graded based on how they execute on xspim running in the CS Department computers. You are to submit your program electronically using the the turnin coomand with the project name of mp1. Your program must be well commented and should use blank lines to separate logical sections of your code.

Alternative Register Names in SPIM

This is from Figure 9.9 in the Goodman&Miller text
Register Name	Alternative Name	Description
$0		the value 0
$1	$at	reserved by the assembler
$2 - $3	$v0 - $v1	expression evaluation and function results
$4 - $7	$a0 - $a3	the first four parameters - not preserved across procedure calls
$8 - $15	$t0 - $t7	temporaries - not preserved across procedure calls
$16 - $23	$s0 - $s7	saved values - preserved across procedure calls
$24 - $25	$t8 - $t9	temporaries - not preserved across procedure calls
$26 - $27	$k0 - $k	reserved for use by the operating system
$28	$gp	global pointer
$29	$sp	stack pointer
$30	$s8	saved value - preserved across procedure calls
$31	$ra	return address
$f0 - $f2		floating point function results
$f4 - $f10		temporaries - not preserved across procedure calls
$f12 - $f14		the first two floating point parameters - not preserved across procedure calls
$f16 - $18		temporaries - not preserved across procedure calls
$f20 - $f30		saved values - preserved across procedure calls

Input and Output in SPIM

I/O is done in SPIM using the syscall opcode. For each I/O operation, a specific value is placed in the $2 register (also referred to as $v0). Depending on the I/O operation to be performed, other registers may be involved. The following table lists the possible syscall services.

Service	System Call Code placed in $2/$v0	Arguments	Results
print_int	1	$a0 = integer
print_float	2	$f12 = float
print_double	3	$f12 = double
print_string	4	$a0 = address of string
read_int	5		integer (in $v0)
read_float	6		float (in $f0)
read_double	7		double (in $f0)
read_string	8	$a0 = address of string buffer $a1 = length of string buffer
sbrk	9	$a0 = amount	address (in $v0)
exit	10

The print_string service expects the address to start a null-terminated character string. The directive .asciiz creates a null-terminated character string.
The read_int, read_float and read_double services read an entire line of input up to and including the newline character.
The read_string service has the same semantices as the UNIX library routine fgets.
- It reads up to n-1 characters into a buffer and terminates the string with a null character.
- If fewer than n-1 characters are in the current line, it reads up to and including the newline and terminates the string with a null character.
The sbrk service returns the address to a block of memory containing n additional bytes. This would be used for dynamic memory allocation.
The exit service stops a program from running.

Spim Input/Output Example Program

The following special characters are used with character strings in SPIM. The special characters follow the C language convention:

Character Encoding Sequence
Newline \n
Tab \t
Double quote \"