Chapter 2. The processor
24
Figure 2.7: ALU simulating an addi operation
the next adder as well as the proper adders required when executing subword additions.
For example if the addition is performed between 4 32bit size words, the fourth adders’
carry out will be driven to the first adder and the 8th adder’s carry out will be driven to
the 5th adder as displayed in Figure
and
. This carry out propagation is required
only for the modular addition where the carry out is fed back to the carry in. The adder
can perform signed/unsigned and modular/non-modular additions/subtractions.
The comparator performs all the basic comparisons(equality, comparison, minimum,
maximum) between the two 64bit words as well as between all the subwords separately
depending on the instruction.
Figure
displays an example of the simulation of the ALU. Presenting here every
single instruction and subword combination is not possible, however all instructions have
been simulated and verified. Appendix D provides a full list with all the components,
including the ALU, and their signal encoding according to the instructions; refer to this
table to set the proper signals in the simulations.
2.5.2
The Multiplier
The multiplier performs all the basic multiplying instructions. It can perform signed or
unsigned multiplications between 16bit size words. These words can be either the odd