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Program requests and reads in exactly one character, which represents an exponent. The program calculates the base 2 power, and then prints the calculated value.

# Assignment 2: Powers of 2 Assembly
# by Hanly De Los Santos
# Copyright ©2009, Hanly De Los Santos. All Rights Reserved.
# http://www.hdelossantos.com
#
# Program requests and reads in exactly one character,
# which represents an exponent. The program calculates
# the base 2 power, and then prints the calculated value.
#
		.text
		.globl __start 

# Load acceptable ascii decimal value for number ranges
# since input is read in as a character.
__start: 	li	$8, 48		# This is the decimal equivalent of 0
		li	$9, 57		# This is the decimal equivalent of 9

# Prompt the user for input.
		la	$11, prompt
		puts	$11

# Grab user input and store value in register 10
		getc	$10
		la	$11, newline	# Create new line
		lb	$17, ($11)
		putc	$17
		add	$15, $10, 0	# Make copy of contents of r10 in r15.
		sub	$15, $15, 48	# Convert char to int.

# Test to determine whether or not the user input value is acceptable.
# If user input is less than 48 and greater than 57, branches to
# exit_donothing, where the user is told the input is wrong.
		bgt	$10, $9, exit_donothing
		blt	$10, $8, exit_donothing
		sub	$10, $10, 48	# After check, convert char to int.

# If the value was acceptable, proceed to calculate the power.
# Checks to determine if $10 is 0 or 1 since they are special cases.
# If value is 0 or 1, branches to their respective prints.
		li	$11, 2		# Load multiplier value into $11.
		beqz	$10, result_zero
		li	$17, 1
		beq	$10, $17, result1
		li	$17, 2		# Initialize 17 to multiplier value
		sub	$10, $10, 1	# Since 2^1 has been taken care of

# Multiplication loop. While $10 != 0 keep multiplying $17 by 2, and
# decrement $10 by 1 every time this is done. Once $10 == 0, print_result.
multiply_for:	mult	$17, $11
		mflo	$17
		sub	$10, $10, 1
		bnez	$10, multiply_for
		beqz	$10, print_result

# Display the value of the exponential function.
print_result:	la	$14, str3
		puts	$14		# Print out "2^"
		li	$v0, 1
		move	$a0, $15
		syscall			# Print out value of user input.
		la	$16, str4
		puts	$16		# Print out '='
		li	$v0, 1
		move	$a0, $17
		syscall			# Print out result.
		la	$13, newline	# Create new line
		lb	$12, ($13)
		putc	$12
		b	endofline

# Print out the result for 2 to the power of 0 (special case).
result_zero:	la	$14, str3
		puts	$14		# Print out "2^"
		li	$v0, 1
		move	$a0, $15
		syscall			# Print out value of user input.
		la	$16, str4
		puts	$16		# Print out '='
		li	$v0, 1
		li	$a0, 1
		syscall
		la	$13, newline	# Create new line
		lb	$12, ($13)
		putc	$12
		b	endofline

# Print the result for 2 to the power of 1 (special case).
result1:	la	$14, str3
		puts	$14		# Print out "2^"
		li	$v0, 1
		move	$a0, $15
		syscall			# Print out value of user input.
		la	$16, str4
		puts	$16		# Print out '='
		li	$v0, 1
		li	$a0, 2
		syscall
		la	$13, newline	# Create new line
		lb	$12, ($13)
		putc	$12
		b	endofline

# If the input is not appropriate, print str2 and exit.
exit_donothing:	la	$13, str2
		puts	$13
		b	endofline

# This is the exit statement.
endofline:	done		# END OF PROGRAM

# Define strings which will be used throughout program
		.data
prompt:		.asciiz "Enter a digit '0'-'9': "
str2:		.asciiz "Bad user input.  Quitting.\n"
str3:		.asciiz "2^"
str4:		.asciiz " = "
newline:  	.byte  '\n'

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