CIS 020 Assembly Programming

CIS 020 Assembly Programming

Chapter 18 - Sub-routines and

Sub-program Linkage

© John Urrutia 2012, All Rights Reserved.5/27/2012 1

ObjectivesUnderstand the Types of SubroutinesDescribe how to create and invoke

SubroutinesDescribe how communications with

Subroutines workDescribe the differences and similarities

between Subroutines and SubprogramsDemonstrate understanding of Subprogram

Linkage and Communications

© John Urrutia 2012, All Rights Reserved. 25/27/2012

SubroutinesGenerally 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


SubroutinesDesignated Flowchart symbol


CALLSubroutin

e

Subroutine

Return to CALL

All needed instructions

Entry Point

Two Types of SubroutinesInternal –

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.


Two Types of SubroutinesExternal –

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.


Subroutine LINKAGETo utilize subroutines

You must address the subroutine entry pointYou must address the next instruction to

execute after the subroutine completes.Linkage between the main routine and

subroutinesUse special branch instructions – BAL & BALRUse the branch register to return to caller– BR


Subroutine LINKAGEBAL – 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


OPERATION OPERANDS10 16

B A L R 5 , S U B R T N 0 1

Subroutine LINKAGEBAL – 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.


Subroutine LINKAGEBALR – 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



B A L R R 1 4 , R 1 5

Subroutine LINKAGEProgram Housekeeping

Keeping track of where we are and where we’re going

Standard Register Linkage ConventionsR1 – Address of the list of data addressesR13 – Address of Calling Program’s Register

Save AreaR14 – Address of Calling Program’s Next

InstructionsR15 – On Entry – Called Program’s Entry Point

Address On Exit – Called Program’s Return

Code© John Urrutia 2012, All Rights Reserved. 115/27/2012

RR – Register to RegisterRR – are the fastest instructions.2 Byte instruction Length with the coding

format

Translates to


RR Op Code R1 R20-7 8-11 12-15 16-19 32-35

First Halfword Second Halfword


M R R 6 , R 8

B'0001 1100 1010 1000'X' 1 C 6 8'

RS – Register to StorageRS – Transfer data between a register and

storage.4 Byte instruction Length with the coding

format

Translates to© John Urrutia 2012, All Rights Reserved. 145/27/2012

B'1001 0000 1110 1100 1101 0000 0000 1100'X' 9 0 E C D 0 0 C'


S T M 1 4 , 1 2 , 1 2 ( 1 3 )

RS

Op Code R1 R3 B2 D2Bits 0-7 8-11 12-15 16-19 20-31


RX – Register to Indexed StorageRX – Transfer control by mask to storage

address .4 Byte instruction Length with the coding

format


B'0100 0111 1111 0101 0110 0011 1111 1000'X' 4 7 F 5 6 3 E 8'


B C 1 5 , 1 0 0 0 ( 5 , 6 )

RX

Op Code M1 R3 B2 D2Bits 0-7 8-11 12-15 16-19 20-31


RX – Register to Indexed StorageRX – Transfer data between a register and

storage.4 Byte instruction Length with the coding

format


B'0100 0000 0100 0101 0110 0011 1111 1000'X' 4 0 4 5 6 3 E 8'


S T H 4 , 1 0 0 0 ( 5 , 6 )

RS

Op Code R1 R3 B2 D2Bits 0-7 8-11 12-15 16-19 20-31


SI – Storage ImmediateSI –Data imbedded in instruction to storage.4 Byte instruction Length with the coding

format

Translates to


B‘1001 0101 0110 1111 0110 0011 1111 1000'X' 9 5 6 F 6 3 E 8'


C L I 1 0 0 0 ( 5 , 6 ) , C ' ? '

RS

Op Code I2 B2 D2Bits 0-7 8-15 16-19 20-31


Storage to Storage InstructionsLeast efficient instructionsSS Formats


SS

Op Code B1 D1 B2 D2Bits 0-7 16-19 20-31 32-35 36-47

L1 L2 B1 D1 B2 D28-11 12-15 16-19 20-31 32-35 36-47

L1 I3 B1 D1 B2 D28-11 12-15 16-19 20-31 32-35 36-47

First Halfword Second Halfword Third Halfword

L18-15

SS – Storage to StorageSS – Storage to Storage, Character Format6 Byte instruction Length with the coding

format

Translates to


B‘1101 0010 1111 1111 0101 0000 0000 1100 0110 0000 0000 0000'X' D 2 F F 5 0 0 C 6 0 0 0'


M V C 1 2 ( 2 5 5 , 5 ) , 0( 6 )

SS

Op Code B1 D1 B2 D2Bits 0-7 16-19 20-31 32-35 36-47


L18-15

SS – Storage to StorageSS – Storage to Storage, Packed Decimal

Format6 Byte instruction Length with the coding

format


B‘1111 0010 0011 0110 0101 0000 0000 1100 0110 0000 0000 1000'X' F 2 3 6 5 0 0 C 6 0 0 8'


P A C K 1 2 ( 4 , 5 ) , 8 ( 7 , 6 )

SS

Op Code L1 L2 B1 D1 B2 D2Bits 0-7 8-11 12-15 16-19 20-31 32-35 36-47


SS – Storage to StorageSS – Storage to Storage, Shift Packed

Decimal Format6 Byte instruction Length with the coding

format


B‘1111 0010 0011 0100 0101 0000 0000 1100 0000 0000 0000 0011'X' F 0 3 4 5 0 0 C 6 0 0 3'

OPERATION OPERANDS Shift Rounding10 16 Number Digit

S R P 1 2 ( 4 , 5 ) , 3 ( 0 ) , 5

SS

Op Code L1 I3 B1 D1 B2 D2Bits 0-7 8-11 12-15 16-19 20-31 32-35 36-47


Program Interrupts & the PSWProgram interrupts occur when you tell the

system to do something it can’t.Old School – System simply stops workingToday – 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


Program Interrupts & the PSWThere 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.


Program Interrupts & the PSWThe PSW – Program Status Word

Identifies the state of the system at the time the interrupt occurred.

z390 PSW Consists of the following informationPSW=07050600 800FFFDA D2030000D088 MVC ABEND S0C5


CIS 020 Assembly Programming

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