Some Announcements

• From APSCUF :
  • We are making plans for showing our appreciation.
  • We will be donating all of the food left over to the Edinboro Food Bank on behalf of the students at Edinboro University.
• I extended the homework date to 10/26
• We do not know the make up schedule but for now we will just carry on.
• You should assume that there will be two extra classes at some point.

Getting Ready to build the ALU

• You should read up to and including 2.7 in chapter 2 and 4.4 in chapter 4
• Last time we started to process an array
  • The code was in class1.asm

    .data
    
    grades: .word 60, 70, 80
    size:	.word 3
    endl:	.asciiz  "\n"	
    	
    .text
    
    main:
    	lw 	$t1, size($zero)
    	li	$v0, 1
    	move	$a0, $t1
    	SYSCALL
    	
    	li	$v0, 10
    	SYSCALL
    	

  • What do all the parts do?
  • Notice pseudo instructions in the machine code.
  • Can we extend this to print the entire array?
    • I would use two registers, one for index (i=0;i<size...
    • one for address (i*4 really)
  • Any questions on math instructions
    • add, addu, addi, addiu, sub, subu
    • Why no subi?
    • Is there a limit to the size of the immediates?
  • Getting around the 16 bit immediate limit.
    • lui
      • R[rt] = {imm,16'b0}
      • Op Code f₁₆
      • I type.
    • ori
      • R[rt] = R[rs]|zeroExtenImm
      • Op Code d₁₆
      • I type
    • So If I want the $t0 to contain 1234ABCD₁₆
      • Note: 1234₁₆ = 16³ + 16²*2 + 16 x 3 + 4 = 4660
      • Note: ABCD₁₆ = 16³ * 10 + 16²* 11 + 16 x 12 + 13 = 43981

        lui	$t1, 4660
        ori	$t1, $t1, 43981
        

    • This technique
      • Allows access to the full 2³² range of values.
      • Immediates, addresses, ....
    • Note:
      • This is a trade off
      • We get to keep a 31 bit word, and use all of it
      • And we can keep the instruction size to be one word.
      • But we need to operations if we are working with very large integers, or large jumps
      • And it is harder to code
      • But the assembler takes care of some of this for us.
        • try li $t0, 16777216 (2²⁴)