Page 128 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Introduction to Microprocessors

Notes older PC compatible machines, only 10 bits of address space were decoded for I/O ports and so
there were only 1024 unique port locations; modern PC’s decode all 16 address lines. To read or
write from a hardware device, special port I/O instructions are used. From a software perspective,
that this is a slight disadvantage because more instructions are required to accomplish the same
task.
If you wanted to test one bit on a memory mapped port, there is a single instruction
to test a bit in memory, but for ports you must read the data into a register, then
test the bit.
The way this applies to arcade emulation is that the port I/O handling routines of your CPU
emulator do not need to have special flags or logic for handling reads and writes to the port
address space since every access to a port will be for the purpose of communicating with a
hardware device. Therefore, in the simplest case, the port I/O handler routine will simply be a
switch statement on the port address. Memory mapped I/O is harder to emulate because you
must maintain some sort of flag list for each address to know if it is ROM, RAM or a ‘special’
address referencing a hardware device.

Touch Tablets

ouch tablets are an interesting subject for a case study. On the one hand, they are
simplicity personified. They are just a flat surface that can sense that it has been touched
Tand communicate to the computer the location of where that touch occurred. On the
other hand, they form the basis for an extremely broad and diverse set of physical and logical
manifestations, as well as interaction techniques. Hence, they constitute a rich source for
improving our understanding of input.
In simplest terms, a touch tablet is typically mounted horizontally on a working surface and
operated with a single finger. But from this basic configuration is a broad range of variations.
They can range in size from an inch per side to several feet. They differ in how much pressure
is required before a touch is registered. Some are capable of continuously reporting to the
computer the amount of pressure being applied by the touch. Some are able to be operated
with a stylus as well as a finger, while others are capable of independently sensing the location
(and sometimes pressure) of multiple simultaneous points of touch. And, as is illustrated in
Figure 8.5, they can be integrated into other devices such as a keyboards or mice.
The biggest problem in any discussion around touch tablets stems from confusing them with
touch screens. The problem is legitimate since the differences between the two are not always
a clear cut as one might first think. Both are controlled by touch. With touch screens, the
touch technology is superimposed over a display. But what about the input device. It is
technically a touch screen, since the touch sensor is over a display. On the other hand, it is
more like a touch tablet, since it is not mounted on the primary visual display, and is
horizontally mounted in a tablet-like fashion.

Contd...

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