I have no idea what it does tho.
also found these parameters hardcoded in there:
SetDiskParms : PROCEDURE (driveselect) CLEAN;
DCL driveselect BYTE;
DCL message (32) BYTE DATA ('Peripheral Bus Driver, 300178-01');
DiskParms(driveselect).devicePageSize = 512;
DiskParms(driveselect).sectorsPerTrack = 17;
DiskParms(driveselect).tracksPerCylinder = 4;
DiskParms(driveselect).numCylinders = 306;
DiskParms(driveselect).writeCurrentCyl = 128;
DiskParms(driveselect).preCompCyl = 128;
DiskParms(driveselect).eccBurstLength = 11;
DiskParms(driveselect).interleaveFactor = 6;
DiskParms(driveselect).controlfield = 027h;
DiskParms(driveselect).numSectorsTrans = 1;
DiskParms(driveselect).pagesize = 512;
DiskParms(driveselect).logicalPageSize = 504;
DiskParms(driveselect).maxLogicalPages = 20808;
DiskParms(driveselect).deviceBitMapFID = 02400h;
DiskParms(driveselect).deviceDirectoryFID = 02420h;
DiskParms(driveselect).dirPages = 10;
CALL MOVB (
@message(0),
@DiskParms(driveselect).message(0),32);
END SetDiskParms;