IBM System/7 Functional Characteristics - All about the IBM 1130 ...

FLAGGINGThe condition of a track surface is indicated to the disk storage module, and thus to thecontrolling program, by track flags recorded as part of every sector identifier on a track.The track flags are two indicator bits that are set to represent one of four possible surfaceconditions: good original, defective original, good alternate, and defective alternate.Cylinders 1 to 3 are used for alternate tracks when a track in cylinders 4 through 202 isfound to be defective. Figure 15-8 shows the correlation between sector identifier bytesrecorded on the disk and the control information word used by read and write commands.0 1 2 9 10 11 15System/7 controlinformation wordF C T S11111111111106 70 7 05 7Sector ID bytesDefective track —IAlternate trackIII L III 11110 = Upper track1 = Lower trackFlag byte Cylinder byte Sector byteTwo cycliccheck bytesFigure 15-8. Correlation between System/7 word in storage and sector ID on disk15-26 GA34-0003Bits 6 and 7 in the flag byte of the sector identifier in each sector are used to indicatethe surface condition of the track in which the sector resides. Bit 6 alone indicates thatthe track is defective, and bit 7 alone indicates that the track is an alternate. When bothbits equal 0, the track is an original "good" track. Both bits set to 1 indicates a defectivealternate track. A defective track in cylinder 0 requires disk repair if used for IPL data.When a track with a bad spot is discovered, it must be marked defective and the programmust assign an alternate track to replace the entire defective track. A format track operationmust first be performed on the assigned alternate track to identify it as an alternate.The formatting operation writes sector identifiers with flag bits 6 and 7 equal to 0 and 1,respectively, and with C and S bytes containing the identifiers from the defective track.Then, any recoverable data from the defective track must be written on the correspondingsectors of the alternate track. Finally, the defective track must be reformatted to identifyit as a defective track. This reformatting operation writes sector identifiers with fin hits6 and 7 equal to 1 and 0, respectively, and with C and S bytes containing the identifiersfrom the alternate track.Should an alternate track be defective, it should be reformatted to identify it as defective.This is done by writing sector identifiers with flag bits 6 and 7 both equal to 1, and with Cand S bytes containing the same identifiers that they originally held.The track identifier fields are summarized as follows:Good: Bits 6 and 7 of the F byte are both 0. The C and S bytes contain the cylinder,track, and sector addresses that are correct for that track.Defective: Bit 6 is 1 and bit 7 is 0 in the F byte. The C and S bytes contain the cylinder,track, and sector addresses from the alternate track.Good alternate: Bit 6 is 0 and bit 7 is 1 in the F byte. The C and S bytes contain thecylinder, track, and sector addresses from the defective track.Defective alternate: Bits 6 and 7 of the F byte are both 1. The C and S bytes contain thecylinder, track, and sector addresses originally assigned to them.