The pieces of an ASSEMBLER program

• The CSECT statement

  • Format 1: label CSECT

  • Format 2: label CSECT , comment

  • defines the beginning of a Control SECTion (equivalent to a function, subroutine, or a procedure)

 

• The END statement

• Format: END label

• Defines the end of what should be assembled.

• Last line of an assembler program.

• label is usually label on the main or 1^st CSECT. If specified, then the CSECT named label is the entry point of the program.

 

• Last line of EXECUTABLE code:

• BCR B’1111’,14

• returns control back to the operating system

Linkage Conventions:

1. On entry to an assembler program, R15 contains the address of the entry point (ie. the beginning of the program). R15 may be used as a base register.

2. On entry to an assembler program, R14 contains the return address, which is an address in the operating system to branch to when the program is finished.

Address          Code to execute
 000000          EXMPL1  CSECT
 000000                  L     5,24(,15)
 000004                  A     5,28(,15)
 000008                  ST    5,36(,15)
 00000C                  L     4,32(,15)
 000010                  SR    4,5
 000012                  ST    4,40(,15)
 000016                  BCR   B’1111’,14
 000018          NUM1    DC    F’15’
 00001C          NUM2    DC    F’7’
 000020          NUM3    DC    F’8’
 000024          RESULT1 DS    F
 000028          RESULT2 DS    F
                         END   EXMPL1

Wouldn’t it have been nice to be able to use the labels NUM1, NUM2, etc...?

• Implicit address – an address that the assembler will be required to convert to an explicit address

• Use a label (usually on a DC or DS in storage) rather than a D(B) or D(X,B) address when coding an instruction

• Format 1: label

• Format 2: label+n -- where n is a decimal displacement

• Format 3: label(R) -- where R is the # of the index register

• Format 4: label+n(R) -- where n is decimal displacement, R is index reg

• Must first establish addressibility (supply the assembler with the base address) with the USING statement.

  • Format: USING label,R

  • R – register # that contains a valid base address

  • label – label in the program that corresponds to base address in R

  • USING does not take up any space.

Address          Code to execute
 000000          EXMPL1  CSECT
 000000                  USING EXMPL1,15
 000000                  L     5,NUM1
 000004                  A     5,NUM2
 000008                  ST    5,RESULT1
 00000C                  L     4,NUM3
 000010                  SR    4,5
 000012                  ST    4,RESULT2
 000016                  BCR   B’1111’,14
 000018          NUM1    DC    F’15’
 00001C          NUM2    DC    F’7’
 000020          NUM3    DC    F’8’
 000024          RESULT1 DS    F
 000028          RESULT2 DS    F
                         END   EXMPL1