IO SPARC

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I/O SPARC
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• A SPARC program in user mode communicates with
  I/O devices through OS
• To do I/O a program makes a system call

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System Components
CPU
bus
Controller Registers - data register - status
register - command register
Memory I/O buffer
Controller Data Status command
Controller Data Status command

• Access to Controller Registers
• memory-mapped
• Isolated I/O

Device
Device
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Interrupt-driven I/O

• CPU can do something else when the I/O controller
  and devices are busy
• Controller sends an external interrupt to signal
  that the device is ready
• Currently executing program stops
• processor enters the OS
• Start executing an interrupt service routine
  (ISR)
• Software routine
• The instructions in ISR are fetched, decoded, and
  executed

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Interrupts

• External Interrupt
• Controller sends an external interrupt to signal
  that the device is ready
• Internal Interrupt
• Divide by zero, unaligned memory access

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Trap

• To protect OS, calls to OS must be made by a
  special instruction that causes an interrupt
• Trap (ta, te, )in SPARC
• an interrupt must save a return address and
  information on the processor state to allow the
  interrupted program to be resumed later
• save the program counter (PC)
• save the processor state register (PSR)

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• trap sequence
• 1. ET bit in PSR cleared so that further traps
  are disabled
• 2. S bit saved into PS bit and S bit set gt
  places processor in supervisor mode
• 3. CWP incremented to give trap handler new set
  of local registers (you can only use the eight
  local registers since the out registers are not
  guaranteed to be mapped to physical registers w/o
  window overflow processing also the in registers
  and global registers might be in use by
  interrupted process)
• 4. pc, and npc are saved into l1 and l2
• 5. trap type field placed into TBR and pc TBR,
  npc TBR4

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Trap Example

• Opening and printing a file
• define(READ, 3)
• define(OPEN, 5)
• ...
• set filename,o0
• clr o1
• clr o2
• mov OPEN,g1
• ta 0 ! fd
  open( char filename, o_flag, mode )
• mov o0,fd_r
• cmp fd_r,-1 ! if -1
  returned then file was not opened ...
• mov fd_r,o0
• set buf,o1
• mov 1,o2
• mov READ,g1
• ta 0 !
  n_read read( fd, buf, 1 )
• cmp o0,0 ! if 0
  returned then there are no characters
• be done ! left in
  the file gt EOF