The internal processor (calculator) state which the user sees is very important. Should there be a single register (accumulator), multiple internal registers, or a stack organized memory? Also, perhaps the calculator should be programmable, providing the capability to store the operations to be performed. Adding program capability to the calculator offers more interesting possibilities. By being programmable, both multiply and divide might. be programmed rather than hardwired. The design should have operations which are accessible directly and via the program. Conditional operations are necessary for a programmable calculator. The programmability introduces several other problems: (1) How is a program loaded into memory? and (2) if inputs must go through. a BCD to binary conversion, then instructions which utilize addresses (in binary form) become awkward to input in BCD.
PROBLEMS
1. What are the cost and performance for a minimum non-programmable calculator which will just add, subtract, and multiply?
2. Design (1) and compare with a more elaborate calculator which still has only the three operations.
3. Design a programmable calculator which is based on principles similar to those used in the computers of this chapter.
KEYWORDS: DMar, computer
At the end of Chapter 2 a new module, the DMar, was introduced. Since it has a 16-word scratchpad memory built in, one can build a simple RTM computer utilizing only a DMar, a T(lights and switches), and a Kbus for the data part. A partially completed (approximately 1/2) ISP description of such a computer, the Crtm-2/2, is given in Figure Crtm-2/2-1. The following problems are based on this computer.
PROBLEMS
1. Finish the ISP description of the half-finished Crtm-2/2 and then implement it using RTM's, thus making Crtm-2.
2. Notice that the skip instructions skip two words rather than one. Why is this so? (Hint: Try writing some non-trival programs for this machine.)
3. Design a version of Crtm-2 that only skips one word on skip instructions, yet is just as generally programmable as the version that skips two words. Compare the compactness of similar programs in the two machines. Compare their cost; their speed.
4. Notice that the effective address of the Crtm-2/2 instruction word is 8-bits long. Design a version of the machine with a 256-word memory. How are the 16 words of the DMar scratchpad utilized to the best advantage in such a computer? Compare the cost and performance of this machine with those described earlier in this chapter.
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