Andrew S. Tanenbaum (OS) Edition 4 Exercise 5 Question 41 (Page No. 433)

Question

A bitmap terminal contains $1600$ by $1200$ pixels. To scroll a window, the CPU (or controller) must move all the lines of text upward by copying their bits from one part of the video RAM to another. If a particular window is $80$ lines high by $80$ characters wide $(6400$ characters, total$),$ and a character’s box is $8$ pixels wide by $16$ pixels high, how long does it take to scroll the whole window at a copying rate of $50\: nsec$ per byte? If all lines are $80$ characters long, what is the equivalent baud rate of the terminal? Putting a character on the screen takes $5\: \mu sec.$ How many lines per second can be displayed?

Answer 1

Total lines of characters required to scroll the window $= 79\;\text{lines of }80\; \text{characters} = 6320 \; \text{characters} $

Time taken to copy $1$ character ($16$ bytes) $= 800\;\text{nsec}$

$\therefore$ Time taken by the complete window$ = 5.056 \; \text{msec}$

Time taken to write $80 $ characters to the screen $= 400\;\text{nsec}$

$\therefore $ Time taken for displaying and scrolling a new line $=5.456 \; \text{msec} = 183.2 \;\frac{\text{lines}}{\text{sec}}$