Nucleus Load Module Loader

We have already developed code that can read the Volume Label, search the VTOC, locate a PDS directory member, and read the data associated with that member. Now we can write some code to load the contents of a load module into memory.

*
         SLR   R3,R3
         ICM   R3,B'0011',CCWREAD+6  LENGTH ATTEMPTED TO READ
         SLR   R1,R1
         ICM   R1,B'0011',IORB+(IORCSW-IOR)+6  GET RESUDUAL LENGTH
         SR    R3,R1              CALC ACTUAL BLOCK LENGTH
*
         B     LOADNUC            LOAD NUCLES LOADMODULE

In our BREAD routine from DISK3 we will calculate the length of the record just read and place the length into Register 3 before branching to LOADNUC.

LOADNUC  DS    0H
         LA    R4,INBUF           POINT TO RECORD DATA
*
         CLI   TEXTSW,0           PROCESSING A TEXT RECORD?
         BNE   TEXT               YES - BRANCH
*
         CLI   0(R4),X'01'        CTL RECORD ?
         BE    CTL
*
         CLI   0(R4),X'05'        CTL RECORD ?
         BE    CTL
*
         CLI   0(R4),X'0D'        CTL RECORD ?
         BE    CTL
*
         CLI   0(R4),X'03'        CTL+RLD RECORD ?
         BE    CTL
*
         CLI   0(R4),X'07'        CTL+RLD RECORD ?
         BE    CTL
*
         CLI   0(R4),X'0F'        CTL+RLD RECORD ?
         BE    CTL
*
         CLI   0(R4),X'02'        RLD RECORD ?
         BE    RLD
*
         CLI   0(R4),X'06'        RLD RECORD ?
         BE    RLD
*
         CLI   0(R4),X'0E'        RLD RECORD ?
         BE    RLD
*
         B     BREAD              SKIP RECORD AND READ NEXT'
*

We start by inspecting the first byte of the record to determine the type of record it is.  We are only concerned with three types of records: Control (CTL), Relocation (RLD) and Text.  It is possible a record may be both a Control and Relocation.  A Text record follows a Control record and the first byte contains data not a record type.  If the record type doesn’t indicate a Control or Relocation record then we can ignore it.  We use TEXTSW as an indicator this record is a Text record.

CTL      DS    0H
         MVI   TEXTSW,1           NEXT RECORD IS A TEXT RECORD
*
         SLR   R1,R1              ZERO R1
         ICM   R1,B'0011',9(R4)   GET LOAD ADDRESS
         ST    R1,TEXTOFF         SAVE OFFSET
         ICM   R1,B'0111',13(R4)  GET LENGTH
         ST    R1,TEXTLEN         SAVE LENGTH
*
         TM    0(R4),X'02'        RLD ALSO
         BO    RLD                YES - BRANCH
*
         B     BREAD              GO READ NEXT RECORD
*

In processing a Control record we start by setting TEXTSW to indicate the next record read will be a Text record.  We then get the address the data is to be loaded at and save it in TEXTOFF and the length of the data which is saved in TEXTLEN.  We check to see if the Control record also contains Relocation data.  If it does we can continue on to RLD else we read the next record.

TEXT     DS    0H
         L     R2,TEXTOFF         GET TEXT OFFSET
         L     R3,TEXTLEN         GET TEXT LENGTH
         LR    R5,R3              COPY LENGTH
*                                 R4 HAD TEXT RECORD ADDRESS
         MVCL  R2,R4              COPY THE DATA
*
         MVI   TEXTSW,0           RESET TEXT SWITCH
         B     BREAD              PROCESS NEXT RECORD
*

Processing of a Text record is straight forward.  We recall the offset and the length of the data and use MVCL to copy it into place.  We then reset TEXTSW to zero and read the next record.

RLD      DS    0H
         B     BREAD              NOT RELOCATING NUCLEUS

Normally when loading a Load Module we have to deal with relocatable symbols.  The load module is based off an address of zero and gets loaded into a higher address.  We would use the RLD entries to add the load address to the value in the relocatable symbol.  Sine we are loading the Nucleus at location zero our relocation offset is zero.  This means we can ignore RLD data for loading the Nucleus.  We simply loop back to read the next record.

BREAD030 DS    0H
*+++++++ EOF READING DATA ++++++++
         LPSW  0                  LOAD NEW PSW
*                                     TO TRANSFER CONTROL TO NUCLEUS

When we get an End of File condition we transfer control to the newly loaded Nucleus by executing a LPSW to load a new PSW from location zero.  The work of the IPL loader is complete.

The memory size, relocation to high memory, and Load Module loader are all implemented in DBOOT3.  It can be run using the Absolute Loader.

DBOOT3 Source Code

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