VTOC

Document Revision: 26h1.0
Source: arch/s390x/init/zxfl/common/dasd_vtoc.c

1. What Is the VTOC?

The Volume Table of Contents (VTOC) is the directory of a z/Architecture DASD volume. It is an IBM-defined on-disk structure that maps dataset names to their physical extents (cylinder/head ranges on ECKD, or block ranges on FBA).

The VTOC begins at a fixed location recorded in the DASD label (Format-4 DSCB at cylinder 0, head 0, record 3 on ECKD). ZXFL reads the VTOC to locate the kernel and loader datasets by name.

2. DSCB Types

The VTOC consists of Data Set Control Blocks (DSCBs), each 140 bytes. ZXFL uses two types:

3. Dataset Lookup

dasd_find_dataset(schid, name, &ext)
  │
  ├─ Read F4DSCB (C=0, H=0, R=3) → get VTOC start C/H and size
  ├─ For each DSCB in VTOC:
  │    ├─ Read record
  │    ├─ Check format byte
  │    ├─ If F1DSCB: compare DS1DSNAM (44-byte EBCDIC name) to target
  │    └─ If match: extract extent list from DS1EXT1..DS1EXT3
  └─ Return first extent (cylinder/head start + end)

Dataset names are stored in EBCDIC on disk. ZXFL converts the search name from ASCII to EBCDIC before comparison using ebcdic_ascii_to_ebcdic().

4. Extent Structure

Each extent describes a contiguous range of tracks:

struct extent {
    uint16_t  cyl_start;   // starting cylinder
    uint16_t  head_start;  // starting head
    uint16_t  cyl_end;     // ending cylinder (inclusive)
    uint16_t  head_end;    // ending head (inclusive)
};

A dataset may span up to three extents in its F1DSCB, with additional extents in a chained F3DSCB. ZXFL follows the F3 chain if the dataset requires more than three extents.

5. Sequential Read

After locating a dataset's extents, dasd_read_next() reads tracks sequentially:

for each extent:
    for each track in [cyl_start/head_start .. cyl_end/head_end]:
        Seek → Search R=1 → Read all records on track → append to buffer

The read stops when the buffer is full or all extents are exhausted.