CMPUT Computer Organization and Architecture I1 CMPUT229 - Fall 2003 Topic6: Logic, Multiply and Divide Operations José Nelson Amaral

CMPUT Computer Organization and Architecture I2 Reading Assignment Appendix A: Section A.9 Chapter 4: Section 4.2, 4.3, 4.4 parts of sections of

CMPUT Computer Organization and Architecture I3 Operating with Unsigned Numbers If a byte is loaded into a register using the lb instruction, the byte will be sign-extended to 32 bits before it is written into the destination register. For instance, if the byte 0x84 is stored in position 0x and the following instruction is executed: lb $t0, 0($gp) # $gp = 0x What is the value of $t0 after this instruction is executed? $t0 = 0xFFFFFF84 How could we get 0x loaded into $t0 from position 0x ? We need to use the load byte unsigned instruction for that effect: lbu $t0, 0($gp)

CMPUT Computer Organization and Architecture I4 sltu and sltiu Sometimes when comparing two values, we want to treat them as unsigned numbers. For this situation MIPS offers two instructions: set less than unsigned (sltu) and set less than immediate unsigned (sltiu) Example: Assume that the following values are stored in $s0 and $s1 $s0 = 0xFFFF FFFF $s1 = 0x What are the values of $t0 and $t1 after the following instructions are executed? slt$t0, $s0, $s1 sltu$t1, $s0, $s1 $t0 = 1 $t1 = 0

CMPUT Computer Organization and Architecture I5 addu, addiu, and subu MIPS also offers unsigned arithmetic operations that do not cause exceptions on overflow: addu: add unsigned addiu: add immediate unsigned subu: subtract unsigned The only difference between the signed instructions add, addi and sub, and the unsigned ones addu, addiu, and subu, is that the unsigned ones do not generate overflow exceptions. The immediate fields of addiu and sltiu are sign-extended! Pat-Hen. pp. 222 and pp. 230

CMPUT Computer Organization and Architecture I6 Logical Shifts MIPS provides bit shifting operations such as shift left logical (sll), and shift right logical (srl). When a logical shift occurs, the new bits introduced in the word are always zeros. Example: Assume that $s0 = What would $t2 contain after the following instruction is executed? sll$t2, $s0, 8# $t2  $s0 << 8 $t2 = Pat-Hen. pp. 226

CMPUT Computer Organization and Architecture I7 Example of logical shifts Propose a sequence of two MIPS instructions that performs the following binary transformation in $s0 (without affecting any other register): Before: $s0 = bbbb bbbb bbbb bbbb bbbb bbcc cccc ccbb After: $s0 = cccc cccc Solution: sll$s0, $s0, 22# $s0  $s0 << 22 # $s0 = cccc cccc bb srl$s0, $s0, 24# $s0  $s0 >> 24 # $s0 = cccc cccc Pat-Hen. pp. 229

CMPUT Computer Organization and Architecture I8 Integer Multiplication and Division using Shifts Shift operations can be used to perform integer multiplication and division by powers of 2. For example to multiply the integer stored in $t1 by 4, we could use the following instruction: sll $t1, $t1, 2# $t1  $t1 << 2 = 4  $t1 If $t1 contained, for instance 0x : Before: $t1 = = 2 3 = 8 10 After: $t1 = = 2 5 = Similarly, a shift right by 2 is equivalent to an integer division by 4.

CMPUT Computer Organization and Architecture I9 Arithmetic Shifts What happens if we shift the number -16 to the right by 2? = After a shift right logical of 2 we get: What is the decimal value represented by this binary number? +16 = = +1,073,741,820

CMPUT Computer Organization and Architecture I10 Something is amiss! -16 = Thus our logical right shift of -16: by 2 resulted in: +1,073,741,820 = If we want to use shift right as a fast alternative to division by powers of two, we need a different shift operation!

CMPUT Computer Organization and Architecture I11 Shift Right Arithmetic MIPS provides a shift right arithmetic (sra) operation that preserves the sign of the number shifted. If $t0 = 0xFFFF FFF0 = The following instruction: sra$t0, $t0, 2# $t0  $t0 >> a 2 = $t0/4 will produce: $t0 = 0xFFFF FFFC = -4 10

CMPUT Computer Organization and Architecture I12 A Note in Binary-to- Decimal Conversion When converting a positive binary number that has a long sequence of 1’s into decimal, is it useful to use the following observations: We wanted to convert the following binary number to decimal: X =

CMPUT Computer Organization and Architecture I13 A Note in Binary-to- Decimal Conversion When converting a positive binary number that has a long sequence of 1’s into decimal, is it useful to use the following observations: We want to convert the following binary number to decimal: X = = Y = = X = = = Y X = Y - 3 = = 2 10  2 10  X = 1024  1024  = +1,073,741,820

CMPUT Computer Organization and Architecture I14 Variable Shifts Sometimes we want to shift a value by a variable number of bits (computed by an algorithm, for instance). For this cases, MIPS provides the shift left logical variable (sllv), shift right logical variable (srlv), and shift right arithmetic variable (srav) instructions Example: What is the value stored in $s1 after the following instructions are executed? addi$t0, $zero, 4 addi$s0, $zero, -128 srav$s1, $s0, $t0 $s1 = -128/16 = -8

CMPUT Computer Organization and Architecture I15 Bitwise Logic Operations Bitwise logic instructions cause the values stored in two registers to be combined, bit by bit, by a logic operator. MIPS implements or, and, not, nor, xor. It also has immediate versions: andi, ori, xori Example: or $t0, $t1, $t2# $t0  $t1 | $t2 Pat-Hen. pp. 227

CMPUT Computer Organization and Architecture I16 Logic Functions If X=0 then X’=1 If X=1 then X’=0 NOT If A=1 AND B=1 then C=1 otherwise C=0 AND If A=1 OR B=1 then C=1 otherwise C=0 OR Pat-Hen. pp. 231

CMPUT Computer Organization and Architecture I17 Logic Functions If X=1 OR Y=1, but not both of them, then C=1 XOR If X=0 AND Y=0, then C=1 NOR

CMPUT Computer Organization and Architecture I18 Bitwise logic example Example: The following is the code of a leaf procedure. How would you describe what this procedure does? add$v0, $zero, $zero andi$t1, $a0, 3 beq$t1, $zero, end add$v0, $zero, 1 end:jr$ra

CMPUT Computer Organization and Architecture I19 Multiply in MIPS Multiplication in Decimal: 88 x two digits operands four digit product Similarly, the binary multiplication of two 32-bit operands produces a 64 bit product. The MIPS has two special register, Hi and Lo, to store the high and low parts of the 64 bit product generated by a multiply operation. Pat-Hen. pp. 264 The multiplication of two 2-digit operands resulted in a 4-digit product.

CMPUT Computer Organization and Architecture I20 Multiply in MIPS MIPS has a multiply instruction, mult, and a multiply unsigned, multu, instruction. MIPS also provides two special instructions to move data from Hi and Lo to the general purpose registers: move from Lo, mflo, and move from Hi, mfhi. Neither the mult nor the multu instruction detects overflow. Example: mult$s0, $s1# Hi  high[$s0  $s1]; Lo  low[$s0  $s1]; MIPS assembler implements a pseudo assembly instruction that moves the values from Hi and Lo into a register: mul$s0, $s1, $s2# $s0  $s1  $s2

CMPUT Computer Organization and Architecture I21 Integer Division Pat-Hen. pp. 273 Quotient Remainder The divide, div, instruction produces the quotient of the division in Lo and the remainder in Hi. The MIPS divide instruction ignores overflow. The MIPS assembler also provides a three register pseudo divide instruction to place the quotient in an specified register.