Review: communication with the memory
We have just studied the READ bus protocol used to communicate with the memory:
Communication with IO devices
We will now study how the CPU communicates with an IO device:
IO communication is a very complex problem and is solved using both hardware and software (interrupt handler !!)
Background information
IO devices are attached to the computer system:
Background information
An IO device is physically made up as 2 functional parts:
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The "actual" IO device hardware
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In CS355, we study how the computer system exchange data with an IO device through the interfac
The IO interfact and
connection on the
system bus
An IO interface consists of 2 different sets of circuits:
The IO interfact and
connection on the
system bus
The IOinterface contains special purpose registers that are connected to the system bus:
The registers are connected to the system bus in the same way as the memory circuit !!!
The IO interfact and
connection on the
system bus
How does the CPU perform an IO operation (= give a command to an IO device):
The CPU sends the command (= data) to an IO device using the bus protocol !!!
The IO interfact and
connection on the
system bus
How does the CPU input/ouput data from/to an IO device:
The CPU sends the read/write the data from/to an IO device using the bus protocol !!!
The IO interfact and
connection on the
system bus
Because IO devices are relatively slow (compared to CPU), the status register can be read to check for readiness:
The CPU reads the status register from an IO device using the bus protocol !!!
How does
the CPU perform an
IO operation
Example: the CPU wants to print the letter 'A'
How does
the CPU perform an
IO operation
The CPU first writes the ASCII code for 'A' to the data register of the printer:
How does
the CPU perform an
IO operation
CPU then writes the command code for 'output' to the command register of the printer:
The command will start the output device (printer) to output the data in the data register !
How does
the CPU perform an
IO operation
While the printer is busy printing, the status will reflect that the IO device is not ready:
Note: the IO device will usually have a larger buffer that allows the CPU to write more data before the IO device become "not ready"
The naive way to perform
IO communication
Consider the following naive way to print (all the characters in) a file:
while (not EOF)
{
Write next character to the printer's Data register
Write the OUTPUT command to the printer's Command register
Read the printer's Status register
while ( status != READY ) // BUSY wait
Read the printer's Status register
}
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The naive way does work...
However: while the CPU is printing the file, the whole computer will stop working (because the CPU is busy printing the file !!!)
(Printing a large file can take a long time !!!)
Difference between
memory communication and
IO communication
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Hence: (new) off-line communication techniques must be used in the communication between CPU and an IO device whereby the CPU do not have to wait !!
How we will discuss the
IO communication technique
We will discuss the following 3 topics to understand IO communication:
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