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 Device registers needed to control disk drive
o Do you want code that someone else has written controlling your disk drive?
 Maybe they accidentally write to some registers that tell the drive to format
itself.
o Solution: Let operating system code (presumably tested and robust!) to control the disk
drive.
 When you write code that saves data to disk, do you read/write to the device
registers?
 No, the OS does this for you.
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 You already know how to USE traps. Now we will learn how they are implemented.
 What trap routines do we have available on the LC-3?
o 20 (GETC), 21 (OUT), 22 (PUTS), 23 (IN), 25 (HALT)
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 Show drawing of lower part of LC-3 memory (0000-00FF)
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 Why Zero-extended?
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 Show flow through diagram of the LC-3.
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 Show trap vector table, trap code, polling for GETC (TRAP x20)
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 Just example of how to use traps – You already know this!
 What would happen if we have data that we are using in R7 before we call a trap?
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 Note that R1 is stored and loaded. Why?
 Why doesn’t the routine save R0?
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 As an example, when you call TRAP x20 to get a character, you don’t expect data that you have
stored in R1 (or R2-R7) to get trashed.
o But you do expect what is in R0 to be trashed – it will contain the read-in character after
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 Callee = the routine being called – Ex: Service Routine
 Caller = the code doing the actual calling – Ex: User code
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 Lives in user space = not part of the operating system = code that you write
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 Get groups to put solutions to FirstChar and CountChar on the board