DLX asm ASCII upper case to lower case

uppercase_to_lowercase.s

// Registers
// R1: Length of text and decrementing counter (Assignment doesn't indicate that it must be preseved)
// R2: Temp register
// R3: Magic value used to add 32 to all bytes in a word

// Memory
// 1000..1000 + r1 : ASCII input
// 2000..2000 + r1 : ASCII output

START:
	addi	r3, r0, #8224	// Construct magic value with bit 6 and bit 14 set
	slli 	r3, r3, #16		// Shift magic value to upper 16 bit
	addi 	r3, r3, #8224  	// Set bit 6 and bit 14 in lower 16 bit again
							// This creates a bitmask that when added to a
							// word adds 32 to each byte in the word. 32 is
							// the offset between 'A' and 'a' in ASCII
	subi	r1, r1, #1		// Decrement length to compensate for offset by 1 of
							// last source byte in memory vs 1000 + r1
LOOP:
	slei	r2, r1, #-1		// Check if we are done
	bnez	r2, END			// Jump to END if we are
	lw		r2, 1000(r1)	// Load word/4 bytes/ascii characters from memory
	add		r2, r2, r3		// Add magic value to chars
	sw		2000(r1), r2	// Store modified chars in target memory area
	subi	r1, r1, #4		// Subtract length of word = 4 bytes from loop counter
	j		LOOP			// aaaaand repeat ;)
END:
	halt					// Halt when finished


// Some additional thoughts:
// This program adds 32 to all bytes in a word even if r1 indicates that the
// text doesn't fully fill the last word in occupies in memory. I guess that
// this isn't a problem here because a) The length of valid data is known and
// b) This is a 32 bit CPU with 32 bit word size. This means that even when
// working with dynamically allocated memory the memory size would be a multiple
// of 4 bytes.