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Lecture 17 - March 12, 2003
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AGENDA:
0 Admin
1 P5 questions?
2 Ch 3.5: block devices in MINIX
3 Ch 3.6: RAM disks in MINIX

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0 - ADMIN
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Startout Q: What types of operations does a block
  device support?

Due today: 
* RI #5: Covers Ch 3.4-7 (pp. 179-222)

Local friendly Dungeon programming contest this
  Saturday (March 15). 1-3:30pm or so. Details TBA,
  but pencil it in on your calendars.

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1 - P5 QUESTIONS?
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Questions?

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2 - CH 3.5: BLOCK DEVICES IN MINIX
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Questions?

OVERVIEW
Initialization
Then go into infinite loop fielding requests
* get message
* do work and send reply
* sharing common code while calling separate service
  routines in C. See fig 3-17 p. 188.
  - example of threads sharing same code while doing
    different work. This takes it further than you
    would normally think of with threads.


The 6 device driver operations
1) Open - verify device is accessible
2) Close - guarantees that buffered data written out
   to final destination.
3) Read - only returns to caller when data transferred
   to user buffer.
4) Write - may buffer data and return immediately.
5) Ioctl - used to change device parameters. E.g.,
   changing speed for serial line.
6) Scattered_IO - can request the reading/writing of
   multiple blocks at once. This has the potential
   to be more efficient. Give the driver a sorted
   list of blocks to read/write and can specify
   whether optional or mandatory per block.

Run through driver.h (pp. 635-636) + driver.c 
  pp. 636-641.

COMMON BLOCK DEVICE SOFTWARE
The 64K DMA boundary problem
* address is specified by a 16-bit counter plus 4- or
  8-bit latch. When counter overflows and wraps around,
  no carry to latch, so suddenly reference an address
  64K lower.
* discuss how to solve
  - allocate a buffer twice required space
  - makes for some really strange errors if don't
    handle correctly.

DRIVER LIBRARY
Partitioning devices
1) Cheaper to partition a larger device that have
   separate devices.
2) O/S may have file system size limits.
3) Many have multiple file system types.
4) Sometimes convenient to have separate logical device
   as in when copy root file system to ram disk.
Put all the machine-specific partition table info here
  in one place.




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3 - CH 3.6: RAM DISKS IN MINIX
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What is a RAM disk?
Pros and cons?
- could probably run MINIX entirely from RAM disk
- Discuss passing comment about systems that support
  mounted file systems vs. those that don't.

Minor devices
0: /dev/ram - true RAM disk, same size as root file
   system by default

1: /dev/mem - used to read and write physical memory
   addresses, starding at absolute 0 (kelvin). Can
   be useful for system devices, debugging, etc.

2: /dev/kmem - like /dev/mem, but address 0 is the
   first byte of the kernel memory. Used for debugging
   and special programs that need to peek at kernel
   data structures (e.g. ps).

3: /dev/null - high compression level data device.
   Unfortunately, this uses a destructive compression
   algorithm and reduces all input to size of 0 bytes.
   When read from, produces instant end of file.

Look through code pp. 645-649.


COS 421 - Lecture Notes #17		SPRING 2003	
COS421-lect-2003-03-12.doc	Page 1	Printed 12.03.03