Describe 16 bit ALU-00214
This subjective question is related to the book/course
vu cs101 Introduction to Computing.
It can also be found in
vu cs101 Mid Term Solved Past Paper No. 3.
Implementing 16-bit ALU
16-bit ALU can be implemented by cascading together four 4-bit ALUs. These 4-bit ALUs have built in Look-Ahead Carry Generator circuits that eliminate the delay caused by carry bit propagating through the Parallel Adder circuit within the 4-bit ALU circut.
However, when a number of such units are cascaded together to implement large 16-bit and 32-bit ALU, the carry propagating between one unit to the next gets delayed due to the Carry rippling
through multiple 4-bit units. For large 32-bit ALUs, the Carry propagates through 8, 4-bit units delaying the Carry out from the last most significant unit by a factor of 8.The 74XX181 and 74XX381 circumvent the problem by having Group-Carry Look-Ahead.
