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.<\/p>\n
*\r\n SLR R3,R3\r\n ICM R3,B'0011',CCWREAD+6 LENGTH ATTEMPTED TO READ\r\n SLR R1,R1\r\n ICM R1,B'0011',IORB+(IORCSW-IOR)+6 GET RESUDUAL LENGTH\r\n SR R3,R1 CALC ACTUAL BLOCK LENGTH\r\n*\r\n B LOADNUC LOAD NUCLES LOADMODULE<\/pre>\nIn 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.<\/p>\n
LOADNUC DS 0H\r\n LA R4,INBUF POINT TO RECORD DATA\r\n*\r\n CLI TEXTSW,0 PROCESSING A TEXT RECORD?\r\n BNE TEXT YES - BRANCH\r\n*\r\n CLI 0(R4),X'01' CTL RECORD ?\r\n BE CTL\r\n*\r\n CLI 0(R4),X'05' CTL RECORD ?\r\n BE CTL\r\n*\r\n CLI 0(R4),X'0D' CTL RECORD ?\r\n BE CTL\r\n*\r\n CLI 0(R4),X'03' CTL+RLD RECORD ?\r\n BE CTL\r\n*\r\n CLI 0(R4),X'07' CTL+RLD RECORD ?\r\n BE CTL\r\n*\r\n CLI 0(R4),X'0F' CTL+RLD RECORD ?\r\n BE CTL\r\n*\r\n CLI 0(R4),X'02' RLD RECORD ?\r\n BE RLD\r\n*\r\n CLI 0(R4),X'06' RLD RECORD ?\r\n BE RLD\r\n*\r\n CLI 0(R4),X'0E' RLD RECORD ?\r\n BE RLD\r\n*\r\n B BREAD SKIP RECORD AND READ NEXT'\r\n*<\/pre>\nWe start by inspecting the first byte of the record to determine the type of record it is.\u00a0 We are only concerned with three types of records: Control (CTL), Relocation (RLD) and Text.\u00a0 It is possible a record may be both a Control and Relocation.\u00a0 A Text record follows a Control record and the first byte contains data not a record type.\u00a0 If the record type doesn’t indicate a Control or Relocation record then we can ignore it.\u00a0 We use TEXTSW as an indicator this record is a Text record.<\/p>\n
CTL DS 0H\r\n MVI TEXTSW,1 NEXT RECORD IS A TEXT RECORD\r\n*\r\n SLR R1,R1 ZERO R1\r\n ICM R1,B'0011',9(R4) GET LOAD ADDRESS\r\n ST R1,TEXTOFF SAVE OFFSET\r\n ICM R1,B'0111',13(R4) GET LENGTH\r\n ST R1,TEXTLEN SAVE LENGTH\r\n*\r\n TM 0(R4),X'02' RLD ALSO\r\n BO RLD YES - BRANCH\r\n*\r\n B BREAD GO READ NEXT RECORD\r\n*<\/pre>\nIn processing a Control record we start by setting TEXTSW to indicate the next record read will be a Text record.\u00a0 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.\u00a0 We check to see if the Control record also contains Relocation data.\u00a0 If it does we can continue on to RLD else we read the next record.<\/p>\n
TEXT DS 0H\r\n L R2,TEXTOFF GET TEXT OFFSET\r\n L R3,TEXTLEN GET TEXT LENGTH\r\n LR R5,R3 COPY LENGTH\r\n* R4 HAD TEXT RECORD ADDRESS\r\n MVCL R2,R4 COPY THE DATA\r\n*\r\n MVI TEXTSW,0 RESET TEXT SWITCH\r\n B BREAD PROCESS NEXT RECORD\r\n*<\/pre>\nProcessing of a Text record is straight forward.\u00a0 We recall the offset and the length of the data and use MVCL to copy it into place.\u00a0 We then reset TEXTSW to zero and read the next record.<\/p>\n
RLD DS 0H\r\n B BREAD NOT RELOCATING NUCLEUS<\/pre>\nNormally when loading a Load Module we have to deal with relocatable symbols.\u00a0 The load module is based off an address of zero and gets loaded into a higher address.\u00a0 We would use the RLD entries to add the load address to the value in the relocatable symbol.\u00a0 Sine we are loading the Nucleus at location zero our relocation offset is zero.\u00a0 This means we can ignore RLD data for loading the Nucleus.\u00a0 We simply loop back to read the next record.<\/p>\n
BREAD030 DS 0H\r\n*+++++++ EOF READING DATA ++++++++\r\n LPSW 0 LOAD NEW PSW\r\n* TO TRANSFER CONTROL TO NUCLEUS<\/pre>\nWhen 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.\u00a0 The work of the IPL loader is complete.<\/p>\n
The memory size, relocation to high memory, and Load Module loader are all implemented in DBOOT3. \u00a0It can be run using the Absolute Loader.<\/p>\n
DBOOT3 Source Code<\/p>\n
[Next – DASD IPL Records<\/a>]<\/p>\n","protected":false},"excerpt":{"rendered":"
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 … Continue reading