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Alignment Restrictions

• Importance: Word and halfword store and load instructions must be properly aligned for effective data access.
• Alignment Requirements:
  • Word accesses must occur at addresses that are multiples of 4.
  • Halfword accesses must occur at addresses that are multiples of 2.

Overview of Arrays

• Definition: An array is a sequence of data items that are contiguous in memory.

• Example of Byte Array:

  • Declaring an array of bytes:
  • .data Primes: .byte 2, 3, 5, 7
  • Memory mapping:
  • Addresses 1001 0000 to 1001 0003 hold values 02 to 07.

• Example of Word Array:

  • Declaring an array of words:
  • .data Primes: .word 2, 3, 5, 7
  • Memory mapping:
  • Each word spans 4 bytes (e.g., Addresses 1001 0000 to 1001 0003 holds 2; 1001 0004 to 1001 0007 holds 3; etc.).

Loading Values from Arrays

• Loading the First Element:
  • Code to load first element:
  • lw $t0, Primes
  • This translates to $t0 = Primes[0] = 2.
• Loading All Elements:
  • Use offsets to access all elements:
  • lw $t0, Primes+0
  • lw $t1, Primes+4
  • lw $t2, Primes+8
  • lw $t3, Primes+12
  • Indexed as: $t0 = Primes[0]</code>, <code>$t1 = Primes[1], $t2 = Primes[2]</code>, <code>$t3 = Primes[3].

Understanding Pointers

• Definition: A pointer is a variable or register that holds a memory address.
• Setting a Pointer:
  • Example of setting a pointer to an array:
  • la $a0, Primes
  • Now, $a0 contains the address of the Primes array (e.g., 1001 0000).
• Loading Values via Pointers:
  • To load first element using pointer:
  • lw $s0, ($a0)
  • Translates to $s0 = a0[0] = 2.
  • Load all elements with offsets:
  • lw $s0, 0($a0)
  • lw $s1, 4($a0)
  • lw $s2, 8($a0)
  • lw $s3, 12($a0).

Performing Operations with Arrays

• Example Operation:
  • Let x be in $s1</code>, y in <code>$s2, address of Z in $s3 and add Z[5] to y:
  • lw $s1, 20($s3) // Load Z[5]
  • add $s1, $s2, $s1 // Calculate x = y + Z[5]

Array of Halfwords Exercise

• Task: Add three halfwords from an array pointed to by $a1:
  • Code:
  • lhu $v0, 0($a1) // Load first halfword
  • lhu $t0, 2($a1) // Load second halfword
  • add $v0, $v0, $t0 // Add first and second values
  • lhu $t0, 4($a1) // Load third halfword
  • add $v0, $v0, $t0</code> // Total stored in <code>$v0.

Address Calculation Example

• Example with Halfword Array B:
  • Let j be in $s0</code> and address of B in <code>$s1:
  • mul $t0, $s0, 2 // Calculate offset for B[j]
  • add $t1, $s1, $t0 // Obtain address of B[j]
  • lw $s2, ($t1) // Load value from B[j].

Assigning Array Values

• Example of Array Assignment:
  • If A[n] = A[m], code example:
  • mul $t0, $a1, 4 // Calculate offset for A[m]
  • add $t0, $a0, $t0 // Address of A[m]
  • lw $t1, ($t0) // Load A[m]
  • mul $t0, $a2, 4 // Calculate offset for A[n]
  • add $t0, $a0, $t0 // Address of A[n]
  • sw $t1, ($t0) // Store value in A[n].

Instruction Summary

• Instructions Used:
  • Load/Store: li, move, la, lw, sw, lhu, sh, lbu, sb
  • Arithmetic: add, sub, mul, div, neg

Recommended Reading

• Text: "Computer Organization and Design: The Hardware/Software Interface" (5th Edition) by David A. Patterson & John L. Hennessy