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Modern Operating Systems by Herbert Bos and Andrew...
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Page 437
appear on the screen.
Graphics adapters often have powerful 32- or 64-bit CPUs
and up to 4 GB of their own RAM, separate from the computer’s main memory.
Each graphics adapter supports some number of screen sizes. Common sizes
vertical in pixels) are 1280
960, 1600
1200, 1920
1080, 2560
1600, and 3840
2160. Many resolutions in practice are in the ratio of 4:3, which
fits the aspect ratio of NTSC and PAL television sets and thus gives square pixels
on the same monitors used for television sets.
Higher resolutions are intended for
wide-screen monitors whose aspect ratio matches them.
At a resolution of just
1080 (the size of full HD videos), a color display with 24 bits per pixel re-
quires about 6.2 MB of RAM just to hold the image, so with 256 MB or more, the
graphics adapter can hold many images at once.
If the full screen is refreshed 75
times/sec, the video RAM must be capable of delivering data continuously at 445
Output software for GUIs is a massive topic. Many 1500-page books have
been written about the Windows GUI alone (e.g., Petzold, 2013; Rector and New-
comer, 1997; and Simon, 1997).
Clearly, in this section, we can only scratch the
surface and present a few of the underlying concepts.
To make the discussion con-
crete, we will describe the Win32 API, which is supported by all 32-bit versions of
Windows. The output software for other GUIs is roughly comparable in a general
sense, but the details are very different.
The basic item on the screen is a rectangular area called a
. A win-
dow’s position and size are uniquely determined by giving the coordinates (in pix-
els) of two diagonally opposite corners.
A window may contain a title bar, a menu
bar, a tool bar, a vertical scroll bar, and a horizontal scroll bar.
A typical window is
shown in Fig. 5-35. Note that the Windows coordinate system puts the origin in
the upper left-hand corner and has
increase downward, which is different from
the Cartesian coordinates used in mathematics.
When a window is created, the parameters specify whether it can be moved by
the user, resized by the user, or scrolled (by dragging the thumb on the scroll bar)
by the user. The main window produced by most programs can be moved, resized,
and scrolled, which has enormous consequences for the way Windows programs
are written.
In particular, programs must be informed about changes to the size of
their windows and must be prepared to redraw the contents of their windows at any
time, even when they least expect it.
As a consequence, Windows programs are message oriented. User actions in-
volving the keyboard or mouse are captured by Windows and converted into mes-
sages to the program owning the window being addressed. Each program has a
message queue to which messages relating to all its windows are sent. The main
loop of the program consists of fishing out the next message and processing it by
calling an internal procedure for that message type.
In some cases, Windows itself
may call these procedures directly, bypassing the message queue. This model is
quite different from the UNIX model of procedural code that makes system calls to
interact with the operating system. X, however, is event oriented.

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