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Bootable Programs

Bootable Programs. Building an O/S. Basic Requirements of any O/S. Respond to interrupts (all kinds) Preserve user environment Protect users and self from corruption Handle I/O errors Handle user errors. Responding to ANY interrupt. Hardware loads a pre-defined state vector

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Bootable Programs

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  1. Bootable Programs Building an O/S

  2. Basic Requirements of any O/S • Respond to interrupts (all kinds) • Preserve user environment • Protect users and self from corruption • Handle I/O errors • Handle user errors

  3. Responding to ANY interrupt • Hardware loads a pre-defined state vector • Context switch occurs • NSI is inside kernel at primary interrupt handler • Primary handler determines type of interrupt • I/O completion • Service call • Program error • Paging/addressing exception • Save user’s environment (stack, registers) in process’s PCB or thread’s TCB • Branch to detail interrupt handler

  4. Detail interrupt handler • Take required action • All instructions valid • Must not cause another interrupt • Therefore must not do I/O - one exception: • Handling I/O errors may require I/O to device • Interrupt for this 2ndary I/O must be deleted • Must allow for all possible (and improbable) errors • Return to common return point

  5. Common return to user • Determine next thread/process to run • New user • Interrupted user • Locate PCB/TCB for selected user • Prep state vector, could be: • Old vector (in reserved RAM) from previously interrupted program • New vector data determined for a new program • Save kernel’s registers in reserved storage • Restore user’s registers & stack from PCB/TCB • Load prepared state vector for user • Context switch occurs

  6. SOS • A Small Operating System • “built-in” user command-line shell • Written in z/390 assembler language • 32-bit mode • Runs in a z/390 virtual machine • You will • Modify it to provide service call handling • Install it as a bootable program • Boot it • Test it

  7. SOS – bootable drive • Virtual device at address 222 • Preformatted as a CMS disk • Simplifies putting programs on the drive • Reserved space for the bootable program (SOS) • Boot loader (IBM supplied) installed by student • Allows seeing disk content from CMS shell

  8. SOS – original contents • Device address discovery • Operator’s terminal • Boot drive • Locate volume TOC • Locates file directory • Displays content of directory in hexadecimal • Terminates

  9. SOS - Lab 1 • Copy the SOSSAMP file to your “a-disk” • Rename it to: SOS • Assemble it using HLASM • Prepare the bootable drive • Boot it

  10. SOS – Lab 2 • Add “shell” code to the single compile • Add state-vector loading to “activate” shell • Shell displays your prompt, waits for input • Shell issues SVC n in response to input • 0<= n <= 7 • See lab assignment for details • SOS gets control and performs service “n” • SOS • returns control to shell • loads “terminate” state vector

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