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Arithmetic / Logic Unit – ALU Design CSE 675.02: Introduction to Computer Architecture Arithmetic / Logic Unit – ALU Design Presentation F Reading Assignment: B5, 3.4 Slides by Gojko Babi 32-bit ALU ALU Control A 32 Result 32-bit 32 ALU Zero Overflow Carry out B 32 • Our ALU should be able to perform functions: – logical and function – logical or function – arithmetic add function – arithmetic subtract function – arithmetic slt (set-less-then) function – logical nor function • ALU control lines define a function to be performed on A and B. g. babic Presentation F 2 1 Functioning of 32-bit ALU ALU Control lines ALU Control 4 Function Ainvert Binvert Operation and 0 0 00 A 32 Result or 0 0 01 32-bit 32 add 0 0 10 ALU Zero Overflow subtract 0 1 10 Carry out B 32 slt 0 1 11 nor 1 1 00 • Result lines provide result of the chosen function applied to values of A and B • Since this ALU operates on 32-bit operands, it is called 32-bit ALU • Zero output indicates if all Result lines have value 0 • Overflow indicates integer overflow of add and subtract functions; for unsigned integers, this overflow indicator does not provide any useful information • Carry out indicates carry out and unsigned integer overflow g. babic Presentation F 3 Designing 32-bit ALU: Beginning 1. Let us start with and function Operation = 0 and 2. Let us now add or function = 1 or a0 0 Result0 b0 1 a1 Result1 0 b1 1 a2 0 Result2 b2 1 a31 0 Result31 b31 1 g. babic 4 2 Designing 32-bit ALU: Principles • A number of functions Operation = 0 and are performed inter- = 1 or nally, but only one a0 and result is chosen for 0 Result0 the output of ALU b0 or 1 a1 and Result1 0 b1 • 32-bit ALU is built or 1 out of 32 identical 1-bit ALU’s a2 and 0 Result2 b2 or 1 a31 and 0 Result31 b31 or 1 g. babic 5 Designing the Adder • 32-bit adder is built out of 32 1-bit adders 1-bit Adder 1-bit Adder Truth Table Input Output a b Carry Sum Carry Figure B.5.2 In Out 0 0 0 0 0 0 0 1 1 0 0 1 0 1 0 From the truth 0 1 1 0 1 table and after 1 0 0 1 0 minimization, we 1 0 1 0 1 can have this 1 1 0 0 1 design for CarryOut 1 1 1 1 1 Figure B.5.5 Figure B.5.3 g. babic Presentation F 6 3 32-bit Adder “0” a0 Cin sum0 b0 + Cout This is a ripple carry adder. a1 Cin sum1 b1 + Cout The key to speeding up addition is determining carry out in the Cin a2 sum2 higher order bits sooner. b2 + Cout Result: Carry look-ahead adder. Cin a31 sum31 b31 + Cout Carry out g. babic Presentation F 7 32-bit ALU With 3 Functions =0 1-bit ALU Operation = 00 and = 01 or = 10 add Figure B.5.6 Figure B.5.7 + carry out CarryOut g. babic Presentation F 8 4 32-bit Subtractor “0”“1” a0 Cin A – B = A + (–B) Result0 b0 + Cout = A + B + 1 a1 Cin Result1 b1 + Cout Cin a2 Result2 b2 + Cout Cin a31 Result31 b31 + Cout CarryOut g. babic Presentation F 9 32-bit Adder / Subtractor binvert “0” a0 Cin Result0 0 b0 + Cout 1 a1 Cin Result1 + b1 0 Cout 1 Cin Binvert = 0 addition a2 Result2 + = 1 subtraction b2 0 Cout 1 Cin a31 Result31 + b31 0 Cout 1 0 CarryOut g. babic 1 10 5 32-bit ALU With 4 Functions 1-bit ALU B i n v e r t O p e r a t i o n B i n v e rt O p e ra t i o n C a r r y I n a 0 C a r r y I n b0 ALU0 R esult0 a 0 C a r r y O u t 1 R e s u l t a 1 C a r r y I n b 0 2 b 1 A L U 1 R e s u l t 1 1 C a r r y O u t Figure B.5.8 C a r r y O u t a 2 C a r r y I n b 2 A L U 2 R e s u l t 2 Control lines C a r r y O u t Function Binvert Operation (1 line) (2 lines) C a r r y I n and 0 00 or 0 01 a 3 1 C a r r y I n R e s u l t 3 1 b 3 1 A L U 3 1 add 0 10 subtract 1 10 0 Carry Out g. babic Presentation F 1 11 2’s Complement Overflow B i n v e r t O p e r a t i o n • 2’s complement overflow C a r r y I n happens: a – if a sum of two positive 0 numbers results in a negative number 1 – if a sum of two negative R e s u l t numbers results in b 0 + 2 a positive number 1 L e s s 3 Carry Out O v e r f l o w O v e r f l o w d e t e c t i o n 1-bit ALU for the most significant bit Other 1-bit ALUs, i.e. non-most significant bit ALUs, are not affected. g. babic Presentation F 12 6 32-bit ALU With 4 Functions and Overflow B i n v e r t O p e r a t i o n a 0 C a r r y I n b 0 A L U 0 R e s u l t 0 C a r r y O u t Control lines a 1 C a r r y I n Function Binvert Operation b 1 A L U 1 R e s u l t 1 (1 line) (2 lines) C a r r y O u t and 0 00 or 0 01 a 2 C a r r y I n b 2 A L U 2 R e s u l t 2 add 0 10 C a r r y O u t subtract 1 10 C a r r y I n a 3 1 C a r r y I n R e s u l t 3 1 b 3 1 A L U 3 1 Missing: slt & nor functions and Zero output O v e r f l o w Carry Out g. babic Presentation F 7 .
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