22-Elec-A4 Digital Systems and Computers · May 2016

Question 5 of 6: Computer system components, microcontrollers and CPU registers

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format. National Exams, May 2016 — 07-Elec-A4 Digital Systems & Computers. Three hours, closed book, one approved Casio or Sharp calculator. Six questions, each worth 12 points; five questions constitute a complete exam. A flip-flop excitation table and a list of Boolean identities are printed on the last page. Every one of the six questions is solved below, because the set is intended as a study resource rather than an exam script.

Reference texts. M. Morris Mano & M. D. Ciletti, Digital Design (6th ed.), ch. 3 (map simplification, prime implicants, hazards), ch. 5–6 (sequential logic, counters); J. F. Wakerly, Digital Design: Principles and Practices (5th ed.), §3.7 (three-state outputs), §4.4 (timing hazards and consensus terms), ch. 8 (counters); C. Hamacher, Z. Vranesic, S. Zaky & N. Manjikian, Computer Organization and Embedded Systems (6th ed.), ch. 1–2 (processor structure, registers), ch. 3 (memory-mapped I/O).

Check: segment-to-pin assignment in Question 6. Figure 6.1 shows the buffer chip driving the eight segment lines a…h from Port B pins PB7–PB0, but does not print which pin drives which segment. Throughout Question 6 the conventional weighting a = PB0, b = PB1, …, g = PB6, h = PB7 (decimal point) is assumed and stated explicitly, exactly as the paper's own rubric invites ("the candidate is urged to submit…a clear statement of any assumptions made"). Every bit pattern below is derived from that one assumption; a different pin order permutes the bits but changes no part of the method.

Question 5: Computer system components, microcontrollers and CPU registers (12 points)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. The question starts from a list of nine candidate items, of which exactly four are the essential components of any stored-program computer, and asks separately about the microprocessor/microcontroller distinction and about four standard CPU register roles.

Find. The four essential components marked from the list; the substantive architectural differences between a general-purpose microprocessor and a microcontroller; and the register associated with each of four described functions.

Approach. Part (a) is settled by asking of each item whether a machine could execute a stored program without it. Parts (b) and (c) are recall questions answered from the standard von Neumann model of processor organisation.

Part (a): mark the four essential components. The test is not whether an item is useful but whether the fetch–decode–execute cycle can proceed without it. A stored-program machine must be able to hold instructions and data, to fetch and execute them, to move information between those two, and to reach the outside world — giving exactly four:

Essential components of a computer system
Item from the list	Essential?	Reason
Processor (CPU)	X	fetches, decodes and executes instructions; contains the ALU and the registers
Memory	X	holds the program and its data — the defining feature of a stored-program machine
Busses (address, data, control)	X	the interconnect that carries addresses, data and timing/control between the other blocks
I/O ports	X	the interface through which the machine communicates with the outside world
Mouse	—	an input peripheral attached through an I/O port
Keyboard	—	an input peripheral attached through an I/O port
Printer	—	an output peripheral
Display monitor	—	an output peripheral
Hard drive	—	secondary storage — a block-addressed peripheral, not main memory

$$\boxed{\text{CPU},\ \text{Memory},\ \text{Busses},\ \text{I/O ports}}$$ The hard drive is the item most often mismarked. It stores information, but the processor cannot execute from it directly; its contents must first be transferred into main memory, and it is reached through an I/O interface exactly like any other peripheral.

Part (b): microprocessor versus microcontroller. The distinction is one of integration and of intended application rather than of instruction-set capability.

General-purpose microprocessor compared with a microcontroller
Aspect	General-purpose microprocessor	Microcontroller
Integration	CPU only; memory, I/O and timers are separate external chips	CPU, RAM, ROM/Flash, I/O ports, timers and often ADCs on a single die — a “computer on a chip”
Buses	address and data buses brought out to pins for external expansion	buses largely internal; pins are devoted to I/O rather than to memory expansion
Memory size	large external memory, gigabytes, with cache and virtual memory support	small on-chip memory, typically kilobytes, with no MMU
Typical peripherals	none on-chip; provided by a chipset	timers, PWM, ADC, UART, SPI, I²C and interrupt controller on-chip
Design objective	maximum throughput and generality; runs an operating system	real-time control, low cost, low power, deterministic response; often bare-metal
Bit handling	word-oriented instruction set	bit-level set/clear/test instructions on port pins
Application	desktop, server, general computing	embedded control — appliances, instrumentation, automotive, the display driver of Question 6

In one sentence: a microprocessor is a processor that needs a system built around it, whereas a microcontroller is the system, optimised for controlling something rather than for computing quickly.

Part (c): identify the registers. Each description names a standard architectural register:

CPU registers and their roles
Description in the question	Register	Function
the address of the next instruction to be executed	Program Counter (PC), also called the instruction pointer	incremented automatically after each fetch; loaded directly by jumps, branches and subroutine calls
the next available location at the top of the stack	Stack Pointer (SP)	adjusted by PUSH/POP and by subroutine call and return, which save and restore the return address
pointing to an array or list of data values in memory	Index register (X or Y on a 68HC-family part; a general-purpose pointer register elsewhere)	supports indexed addressing, so a single instruction can be re-used to walk successive elements
information used at decision-making points (conditional branches)	Condition Code Register (CCR), also called the status or flags register	holds the N, Z, V and C flags set by the ALU; conditional branch instructions test these bits

$$\boxed{\text{PC},\quad \text{SP},\quad \text{Index register (X/Y)},\quad \text{Condition Code / status register}}$$ Note that the accumulator, though the most-used register of all, answers none of these four descriptions — it holds operands and results, not addresses or status.

The three parts are more closely linked than they first appear. The four essential components of part (a) are precisely what a microcontroller integrates onto one die in part (b), and the registers of part (c) are the internal state that lets the CPU of part (a) sequence a program at all: the PC drives the fetch, the index register drives data access, the CCR drives control flow, and the SP makes subroutines possible.

Question 5 — final results
Quantity	Result
(a) essential components	Processor (CPU), Memory, Busses (address/data/control), I/O ports
(a) not essential	Mouse, Keyboard, Printer, Display monitor, Hard drive (all peripherals)
(b) key difference	Microprocessor = CPU alone needing external memory and I/O; microcontroller = CPU + memory + I/O + timers on one chip, for embedded real-time control
(c) next instruction address	Program Counter (PC)
(c) top of stack	Stack Pointer (SP)
(c) pointer to an array/list	Index register (X or Y)
(c) conditional-branch information	Condition Code Register (status/flags: N, Z, V, C)

Topic. Processor organisation: the essential blocks of a stored-program computer, the microprocessor/microcontroller distinction, and the architectural register set.

Key relations. The von Neumann model — processor, memory, input/output, interconnected by address, data and control buses — and the fetch–decode–execute cycle, in which the PC supplies the address, memory returns the instruction, the CCR records the ALU result, and the SP maintains the return-address stack.

Why this works. Every item on the part (a) list either participates in the fetch–decode–execute cycle or is reached through it. That single test separates the four essentials from the five peripherals without any need to memorise a list, and it also explains why the hard drive — despite being storage — falls on the peripheral side: instructions cannot be fetched from it. The same reasoning explains part (c): each register named exists because some step of the cycle needs a piece of state to persist between instructions, so the register set is a direct consequence of the model rather than an arbitrary design choice.

Common pitfall. Marking the hard drive as essential (confusing secondary storage with main memory) or omitting the busses (which are easy to overlook because they are wires rather than chips) are the two ways part (a) is commonly lost. In part (b), a weak answer lists only “a microcontroller is smaller”; the marks are for the architectural consequence — on-chip memory and peripherals, hence fewer external components, deterministic timing and a design optimised for control rather than throughput.

Source. Hamacher, Vranesic, Zaky & Manjikian, Computer Organization and Embedded Systems 6th ed., §1.2 (basic functional units), §2.2 (register structure and addressing modes), ch. 3 (embedded processors and microcontrollers).

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