1. CIS 020 Assembly Programming
Chapter 18 -
Sub-routines and Sub-program Linkage
5/27/2012
© John Urrutia 2012, All Rights Reserved.

2. Objectives Understand the Types of Subroutines
Describe how to create and invoke Subroutines
Describe how communications with Subroutines work
Describe the differences and similarities between Subroutines and Subprograms
Demonstrate understanding of Subprogram Linkage and Communications
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3. Subroutines
Generally a block of instructions that perform a specific function needed from several points within a program.
Provides storage efficiency by creating the specific function once and “Calling” that function from wherever it is needed.
Subroutines are identified by a label at their entry point and generally terminate with a BR instruction
5/27/2012
© John Urrutia 2012, All Rights Reserved.

4. All needed instructions
Subroutines
Entry Point
Designated Flowchart symbol
Subroutine
CALL
Subroutine
All needed instructions
Return to CALL
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5. Two Types of Subroutines
Internal –
Written specifically for this program for use anywhere in this program.
Relies on the supporting environment provided by the main program.
Generally, must be within the address range of the current base register.
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6. Two Types of Subroutines
External –
Written as an independent stand-alone set of reusable code that can be added anywhere into a program.
Generally stored in a system library and copied into your program via a copybook or MACRO.
5/27/2012
© John Urrutia 2012, All Rights Reserved.

7. Subroutine LINKAGE To utilize subroutines
You must address the subroutine entry point
You must address the next instruction to execute after the subroutine completes.
Linkage between the main routine and subroutines
Use special branch instructions – BAL & BALR
Use the branch register to return to caller– BR
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8. Subroutine LINKAGE BAL – Branch and Link
Stores the Address of the instruction following the BAL in the Register of Operand 1 Then
Branches to the Address of Operand 2
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© John Urrutia 2012, All Rights Reserved.

9. Subroutine LINKAGE BAL – Branch and Link
Use caution – inadvertently changing the contents of the register in Operand 1 will have unpredictable results!!!
What do I do if I need to use this register??
Upon entry to the subroutine – Store the Register using the ST instruction
Create a 4 Byte storage area for this purpose.
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© John Urrutia 2012, All Rights Reserved.

10. Subroutine LINKAGE BALR – Branch and Link Register
This instruction is primarily used for calling external subroutines / subprograms.
Stores the Address of the instruction following the BALR in the Register of Operand 1 Then
Branches to the Address of Operand 2
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11. Subroutine LINKAGE Program Housekeeping
Keeping track of where we are and where we’re going
Standard Register Linkage Conventions
R1 – Address of the list of data addresses
R13 – Address of Calling Program’s Register Save Area
R14 – Address of Calling Program’s Next Instructions
R15 – On Entry – Called Program’s Entry Point Address On Exit – Called Program’s Return Code
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12. 5/27/2012
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13. RR – Register to Register
RR – are the fastest instructions.
2 Byte instruction Length with the coding format
Translates to
B' '
X' 1 C '
5/27/2012
© John Urrutia 2012, All Rights Reserved.

14. RS – Register to Storage
RS – Transfer data between a register and storage.
4 Byte instruction Length with the coding format
Translates to
B' '
X' E C D C'
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15. RX – Register to Indexed Storage
RX – Transfer control by mask to storage address .
4 Byte instruction Length with the coding format
Translates to
B' '
X' F E 8'
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© John Urrutia 2012, All Rights Reserved.

16. RX – Register to Indexed Storage
RX – Transfer data between a register and storage.
4 Byte instruction Length with the coding format
Translates to
B' '
X' E 8'
5/27/2012
© John Urrutia 2012, All Rights Reserved.

17. SI – Storage Immediate SI –Data imbedded in instruction to storage.
4 Byte instruction Length with the coding format
Translates to
B‘ '
X' F E 8'
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© John Urrutia 2012, All Rights Reserved.

18. Storage to Storage Instructions
Least efficient instructions
SS Formats
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19. SS – Storage to Storage SS – Storage to Storage, Character Format
6 Byte instruction Length with the coding format
Translates to
B‘ '
X' D F F C '
5/27/2012
© John Urrutia 2012, All Rights Reserved.

20. SS – Storage to Storage SS – Storage to Storage, Packed Decimal Format
6 Byte instruction Length with the coding format
Translates to
B‘ '
X' F C '
5/27/2012
© John Urrutia 2012, All Rights Reserved.

21. SS – Storage to Storage
SS – Storage to Storage, Shift Packed Decimal Format
6 Byte instruction Length with the coding format
Translates to
B‘ '
X' F C '
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© John Urrutia 2012, All Rights Reserved.

22. Program Interrupts & the PSW
Program interrupts occur when you tell the system to do something it can’t.
Old School – System simply stops working
Today – System is built with redundancy and continues to work while suspending your program execution
When an interrupt occurs, generally your program is terminated with an appropriate system code.
There are 5 types of interrupts
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23. Program Interrupts & the PSW
There are 5 types of interrupts
External – caused by an action outside of the system.
I/O problem – external or internal associated with I/O subsystem
Program – caused by a faulty instruction or address
Machine – hardware, faulty circuit or equipment
Supervisor – caused by an error detected by the Supervisor Program.
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24. Program Interrupts & the PSW
The PSW – Program Status Word
Identifies the state of the system at the time the interrupt occurred.
z390 PSW Consists of the following information
PSW= FFFDA D D088 MVC ABEND S0C5
5/27/2012
© John Urrutia 2012, All Rights Reserved.