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CHAPT ER 4 T HE INST RUCT ION SET ARCHIT ECT URE
4.7 Input and Output in Assembly Language
Finally, we come to ways in which an assembly language program can communi- cate with the outside world: input and output IO activities. One way that
communication between IO devices and the rest of the machine can be handled is with special instructions, and with a special IO bus reserved for this purpose.
An alternative method for interacting with IO devices is through the use of memory mapped IO, in which devices occupy sections of the address space
where no ordinary memory exists. Devices are accessed as if they are memory locations, and so there is no need for handling devices with new instructions.
As an example of memory mapped IO, consider again the memory map for the ARC, which is illustrated in Figure 4-20. We see a few new regions of memory,
A stack frame is created for func_2 as a result of
function call at line 04 of program.
g
A stack frame is created for func_1 as a result of
function call at line 19 of program.
h
func_1 places return
value on stack. Line 12 of program.
i
2
32
– 4
Free area
2
32
– 4
sp
Stack 2
32
– 4
Free area sp
func_2 places return
value on stack. Line 20 of program.
j
Program finishes. Stack is restored to its initial configuration. Lines
04 and 05 of program.
k
2
32
– 4
Free area
2
32
– 4
Stack
115
sp Stack
frame for func_2
Free area sp
Stack Stack
sp
10 r15
m n
10 r15
m n
10 15
r15 i
j
func_2 stack frame
func_1 stack frame
115 r15
m n
10
Stack
Free area
Figure 4-19 g-k Continued.
CHAPT ER 4 T HE INST RUCT ION SET ARCHIT ECT URE
143
for two add-in video memory modules and for a
touchscreen
. A touchscreen comes in two forms, photonic and electrical. An illustration of the photonic ver-
sion is shown in Figure 4-21. A matrix of beams covers the screen in the horizon-
tal and vertical dimensions. If the beams are interrupted by a finger for example then the position is determined by the interrupted beams. In an alternative ver-
sion of the touchscreen, the display is covered with a touch sensitive surface. T he user must make contact with the screen in order to register a selection.
Reserved for built-in bootstrap and graphics
routines Add-in video memory 1
IO space 2
16
Stack pointer
System Stack
Top of stack Bottom of stack
Screen Flash Touchscreen x
Touchscreen y 2
24
– 4 2
23
– 4 32 bits
Address Data
2
24
– 1 byte
Add-in video memory 2
2
17
2
19
Working Memory Unused
2
22
FFFFEC
16
FFFFF0
16
FFFFF4
16
Figure 4-20 Memory map for the ARC, showing memory mapping.
LEDs sources
Detector User breaks
beams
Figure 4-21 A user selecting an object on a touchscreen.
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CHAPT ER 4 T HE INST RUCT ION SET ARCHIT ECT URE
T he only real memory occupies the address space between 2
22
and 2
23
– 1. Remember: 2
23
– 4 is the address of the leftmost byte of the highest word in the big-endian format. T he rest of the address space is occupied by other compo-
nents. T he address space between 0 and 2
16
– 1 inclusive contains built-in pro- grams for the power-on bootstrap operation and basic graphics routines. T he
address space between 2
16
and 2
19
– 1 is used for two add-in video memory modules, which we will study in Problem Figure 4.3. Note that valid informa-
tion is available only when the add-in memory modules are physically inserted into the machine.
Finally, the address space between 2
23
and 2
24
– 1 is used for IO devices. For this system, the X and Y coordinates that mark the position where a user has
made a selection are automatically updated in registers that are placed in the memory map. T he registers are accessed by simply reading from the memory
locations where these registers are located. T he “Screen Flash” location causes the screen to flash whenever it is written.
Suppose that we would like to write a simple program that flashes the screen whenever the user changes position. T he flowchart in Figure 4-22 illustrates how
this might be done. T he X and Y registers are first read, and are then compared with the previous X and Y values. If either position has changed, then the screen
is flashed and the previous X and Y values are updated and the process repeats. If neither position has changed, then the process simply repeats. T his is an example
of the programmed IO method of accessing a device. See problem 4.3 at the end of the chapter for a more detailed description.
4.8 Case Study: The Java Virtual M achine ISA
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