Data Types and Literals

Data types:

• Instructions are all 32 bits
• byte(8 bits), halfword (2 bytes), word (4 bytes)
• a character requires 1 byte of storage
• an integer requires 1 word (4 bytes) of storage

Literals:

• numbers entered as is. e.g. 4
• characters enclosed in single quotes. e.g. 'b'
• strings enclosed in double quotes. e.g. "A string"

Registers

• 32 general-purpose registers
• register preceded by $ in assembly language instruction
  two formats for addressing:
  • using register number e.g. $0 through $31
  • using equivalent names e.g. $t1, $sp
• special registers Lo and Hi used to store result of multiplication and division
  • not directly addressable; contents accessed with special instruction mfhi ("move from Hi") and mflo ("move from Lo")
• stack grows from high memory to low memory

See also Britton section 1.9, Sweetman section 2.21, Larus Appendix section A.6

Program Structure

• just plain text file with data declarations, program code (name of file should end in suffix .s to be used with SPIM simulator)
• data declaration section followed by program code section

Data Declarations

• placed in section of program identified with assembler directive .data
• declares variable names used in program; storage allocated in main memory (RAM)

Code

• placed in section of text identified with assembler directive .text
• contains program code (instructions)
• starting point for code execution given label main:
• ending point of main code should use exit system call (see below under System Calls)

Comments

• anything following # on a line
  # This stuff would be considered a comment
• Template for a MIPS assembly language program:

  # Comment giving name of program and description of function
  # Template.s
  # Bare-bones outline of MIPS assembly language program
  
             .data       # variable declarations follow this line
                         # ...
  														
             .text       # instructions follow this line	
  																	
  main:                  # indicates start of code (first instruction to execute)
                         # ...
  									
  # End of program, leave a blank line afterwards to make SPIM happy
  

Data Declarations

format for declarations:

name:	storage_type	value(s)	

• create storage for variable of specified type with given name and specified value
• value(s) usually gives initial value(s); for storage type .space, gives number of spaces to be allocated

Note: labels always followed by colon ( : )

example
	
var1:		.word	3	# create a single integer variable with initial value 3
array1:		.byte	'a','b'	# create a 2-element character array with elements initialized
				#   to  a  and  b
array2:		.space	40	# allocate 40 consecutive bytes, with storage uninitialized
				#   could be used as a 40-element character array, or a
				#   10-element integer array; a comment should indicate which!	

Load / Store Instructions

• RAM access only allowed with load and store instructions
• all other instructions use register operands

load:

	lw	register_destination, RAM_source

#copy word (4 bytes) at source RAM location to destination register.

	lb	register_destination, RAM_source

#copy byte at source RAM location to low-order byte of destination register,
# and sign-extend to higher-order bytes

store word:

	sw	register_source, RAM_destination

#store word in source register into RAM destination

	sb	register_source, RAM_destination

#store byte (low-order) in source register into RAM destination

load immediate:

	li	register_destination, value

#load immediate value into destination register

 

example:
	.data
var1:	.word	23		# declare storage for var1; initial value is 23

	.text
__start:
	lw	$t0, var1		# load contents of RAM location into register $t0:  $t0 = var1
	li	$t1, 5		#  $t1 = 5   ("load immediate")
	sw	$t1, var1		# store contents of register $t1 into RAM:  var1 = $t1
	done

Indirect and Based Addressing

• Used only with load and store instructions

load address:

	la	$t0, var1

• copy RAM address of var1 (presumably a label defined in the program) into register $t0

indirect addressing:

	lw	$t2, ($t0)

• load word at RAM address contained in $t0 into $t2

	sw	$t2, ($t0)

• store word in register $t2 into RAM at address contained in $t0

based or indexed addressing:

	lw	$t2, 4($t0)

• load word at RAM address ($t0+4) into register $t2
• "4" gives offset from address in register $t0

	sw	$t2, -12($t0)

• store word in register $t2 into RAM at address ($t0 - 12)
• negative offsets are fine

Note: based addressing is especially useful for:

• arrays; access elements as offset from base address
• stacks; easy to access elements at offset from stack pointer or frame pointer

 

example

		.data
array1:		.space	12		#  declare 12 bytes of storage to hold array of 3 integers
		.text
__start:	la	$t0, array1		#  load base address of array into register $t0
		li	$t1, 5		#  $t1 = 5   ("load immediate")
		sw $t1, ($t0)		#  first array element set to 5; indirect addressing
		li $t1, 13		#   $t1 = 13
		sw $t1, 4($t0)		#  second array element set to 13
		li $t1, -7		#   $t1 = -7
		sw $t1, 8($t0)		#  third array element set to -7
		done

Arithmetic Instructions

• most use 3 operands
• all operands are registers; no RAM or indirect addressing
• operand size is word (4 bytes)

		add	$t0,$t1,$t2	#  $t0 = $t1 + $t2;   add as signed (2's complement) integers
		sub	$t2,$t3,$t4	#  $t2 = $t3 Ð $t4
		addi	$t2,$t3, 5	#  $t2 = $t3 + 5;   "add immediate" (no sub immediate)
		addu	$t1,$t6,$t7	#  $t1 = $t6 + $t7;   add as unsigned integers
		subu	$t1,$t6,$t7	#  $t1 = $t6 + $t7;   subtract as unsigned integers

		mult	$t3,$t4		#  multiply 32-bit quantities in $t3 and $t4, and store 64-bit
					#  result in special registers Lo and Hi:  (Hi,Lo) = $t3 * $t4
		div	$t5,$t6		#  Lo = $t5 / $t6   (integer quotient)
					#  Hi = $t5 mod $t6   (remainder)
		mfhi	$t0		#  move quantity in special register Hi to $t0:   $t0 = Hi
		mflo	$t1		#  move quantity in special register Lo to $t1:   $t1 = Lo
					#  used to get at result of product or quotient

		move	$t2,$t3	#  $t2 = $t3

Control Structures

Branches

• comparison for conditional branches is built into instruction

		b	target		#  unconditional branch to program label target
		beq	$t0,$t1,target	#  branch to target if  $t0 = $t1
		blt	$t0,$t1,target	#  branch to target if  $t0 < $t1
		ble	$t0,$t1,target	#  branch to target if  $t0 <= $t1
		bgt	$t0,$t1,target	#  branch to target if  $t0 > $t1
		bge	$t0,$t1,target	#  branch to target if  $t0 >= $t1
		bne	$t0,$t1,target	#  branch to target if  $t0 <> $t1

Jumps

		j	target	#  unconditional jump to program label target
		jr	$t3		#  jump to address contained in $t3 ("jump register")

Subroutine Calls

subroutine call: "jump and link" instruction

	jal	sub_label	#  "jump and link"

• copy program counter (return address) to register $ra (return address register)
• jump to program statement at sub_label

subroutine return: "jump register" instruction

	jr	$ra	#  "jump register"

• jump to return address in $ra (stored by jal instruction)

Note: return address stored in register $ra; if subroutine will call other subroutines, or is recursive, return address should be copied from $ra onto stack to preserve it, since jal always places return address in this register and hence will overwrite previous value

System Calls and I/O (SPIM Simulator)

• used to read or print values or strings from input/output window, and indicate program end
• use syscall operating system routine call
• first supply appropriate values in registers $v0 and $a0-$a1
• result value (if any) returned in register $v0

The following table lists the possible syscall services.

• 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.

e.g.   Print out integer value contained in register $t2

		li	$v0, 1			# load appropriate system call code into register $v0;
						# code for printing integer is 1
		move	$a0, $t2		# move integer to be printed into $a0:  $a0 = $t2
		syscall				# call operating system to perform operation


e.g.   Read integer value, store in RAM location with label int_value (presumably declared in data section)

		li	$v0, 5			# load appropriate system call code into register $v0;
						# code for reading integer is 5
		syscall				# call operating system to perform operation
		sw	$v0, int_value		# value read from keyboard returned in register $v0;
						# store this in desired location

e.g.   Print out string (useful for prompts)

		.data
string1		.asciiz	"Print this.\n"		# declaration for string variable, 
						# .asciiz directive makes string null terminated

		.text
main:		li	$v0, 4			# load appropriate system call code into register $v0;
						# code for printing string is 4
		la	$a0, string1		# load address of string to be printed into $a0
		syscall				# call operating system to perform print operation


e.g. To indicate end of program, use exit system call; thus last lines of program should be:

		li	$v0, 10		# system call code for exit = 10
		syscall				# call operating sys