TRS-80 DOS - VTOS 4.0.2 for the Model I - BOOT/SYS Disassembled

BOOT/SYS

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VTOS 4.0.2 BOOT/SYS Disassembly - The Boot Sectors and the System Information Sector (Model I)

BOOT/SYS is the first file of every VTOS diskette: granule 0 of cylinder 0. Its directory record (the first record of sector 2 of the directory cylinder) gives it 4 records, sectors 0-3, so the file is 1024 bytes. It is not a load module. Sector 0 is the boot sector the ROM runs, sector 1 is a boot sector for a boot through a disk driver, sector 2 is the system information sector and sector 3 is a copy of a granule allocation table. The fifth sector of the granule is not part of the file: FORMAT/CMD writes it with the sector number 7CH in its address mark instead of 4 (its 60CDH-60DBH, on side 0 of cylinder 0), so a sector read of cylinder 0, sector 4 does not find it.

When the computer is reset with a diskette in drive 0, the ROM reads cylinder 0, sector 0 into 4200H-42FFH and jumps to 4200H. The boot sector reads sector 4 of the directory cylinder (the cylinder number is byte 2 of the sector, 4202H), checks that the directory record of SYS0/SYS there holds a file, works out where the file's first extent starts, reads it sector by sector into a buffer at 5100H-51FFH, copies each load block to its address and jumps to the transfer address, 4E00H, the reset of SYS0/SYS. A free directory record displays NO SYSTEM; a sector that cannot be read in two tries displays DISK ERROR. Both wait for a line from the keyboard and stop the computer until RESET is pressed.

SYS0/SYS's reset then reads sector 2 over the boot sector at 4200H. It checks the 64 bytes C0H-FFH, displays the serial number in bytes C0H-C7H after the words VTOS SERIAL #, takes byte 01H as the switch that loads the configuration file CONFIG/SYS, and copies the drive code tables of drives 1-7 from bytes 2AH-6FH to 470AH-474FH.

All four sectors are shown at 4200H-42FFH: sectors 0 and 1 where they run, sectors 2 and 3 where SYS0/SYS, SYS2/SYS and SYS6/SYS read the system information sector and the allocation table. Rows of sectors 1-3 carry ids that name the sector (s1-4246H, for example).

FORMAT/CMD on this diskette carries both boot sectors as images (6A00H and 6B00H) and fills in the directory cylinder (byte 2) and the bootstrap stepping rate (42BCH in sector 0, 4219H in sector 1) when it formats a diskette. Its image of sector 0 differs from the sector 0 on this page in six bytes besides those: the patch it carries named FASBOOT2 puts EX (SP),HL at 42BDH-42C0H and 42CBH-42CCH where this sector has PUSH AF / POP AF twice and PUSH BC / POP BC, a longer delay before the controller's status is read.

How Sector 0 Reads a Sector

The Four Sectors

The Drive Code Table

4200H - Sector 0: Boot Sector Entry

When the computer is reset with a diskette in drive 0, the ROM reads cylinder 0, sector 0 into 4200H-42FFH and jumps to 4200H. This sector finds SYS0/SYS through its directory record (the first record of sector 4 of the directory cylinder), reads the file's first extent sector by sector into the buffer at 5100H-51FFH, copies each load block to its address and jumps to the transfer address (4E00H on this diskette). It works the floppy disk controller directly through its memory mapped registers at 37ECH-37EFH and the drive select latch at 37E1H.

422EH - Locate SYS0/SYS on the Diskette

Byte 16H of the directory record is the cylinder of the file's first extent; byte 17H holds the first granule on that cylinder in bits 7-5 and the number of granules less one in bits 4-0 (SYS0/SYS's record on this diskette: 00H and 23H, cylinder 0, granule 1, four granules). This code works the granule into a sector number at 5 sectors a granule and sets up the alternate register set, which does all the reading from here on: Register D' holds the cylinder and Register E' the sector of the next sector to read, Register Pair BC' the pointer into the buffer. Only the first extent is used, read as consecutive sectors.

4242H - Load Record Loop

SYS0/SYS is a load module. Each record starts with a type byte and a length byte. Type 01H is a load block: two address bytes and then the data, the length counting the address bytes, so a length of 2 gives no data and a length of 0 or 1 gives 254 or 255 data bytes. Type 02H is the transfer record. Every other type is skipped by its length. The main register set does the loading: Register Pair HL is the load address and Register B the bytes left in the record.

426DH - Transfer Record: Start SYS0/SYS

The transfer record is the type byte 02H, a length byte and the two byte address SYS0/SYS starts at (4E00H on this diskette, its reset). The jump leaves interrupts off, Index Register IY = 0000H and the stack at 41E0H.

4279H - Fetch the Next Byte of SYS0/SYS

Returns the next byte of the file in Register A. All its work is done in the alternate register set: Register Pair BC' points into the buffer at 5100H-51FFH and Register Pair DE' holds the cylinder and sector of the next sector to read. When the buffer is used up, the next sector is read into it, 10 sectors to a cylinder. The main Register Pairs BC, DE and HL are not changed.

4294H - Display an Error and Stop

Entered at 4294H when a sector cannot be read and at 4297H with Register Pair HL pointing to NO SYSTEM. The text is displayed, the ROM waits for a line from the keyboard and the computer stops; only RESET starts it again.

429EH - Display a Text

Displays the text at Register Pair HL through the ROM, character by character, up to the end byte 03H (not displayed). Register Pair HL is unchanged on return.

42ACH - Read a Sector, Two Tries

Reads cylinder Register D, sector Register E of drive 0 into the buffer at Register Pair BC. The first try is the CALL 42B4H; when it fails, the code falls into 42B4H for the second try, which returns straight to the caller. On return the Z FLAG is set if the sector was read, Register Pair HL holds the buffer address and Register Pair BC points past the last byte stored.

42B4H - Seek and Read One Sector

Seeks the head to cylinder Register D and reads sector Register E into the buffer at Register Pair BC, taking each byte from the controller as it signals DATA REQUEST. Returns with the Z FLAG set if the sector was read without error.

Sector 1: Boot Sector for a Boot Through a Disk Driver

Sector 1 is a second boot sector, shown at 4200H-42FFH where it runs. It is not run by the ROM and no program on this diskette jumps to it: it is entered with Index Register IY pointing to the drive code table of drive 0, whose first three bytes (+00H-+02H) jump to a disk driver. All its reads go through that driver (function 09H, read a sector, the function SYS0/SYS's own driver at 4600H serves with the controller's Read Sector command). It takes the geometry from the drive code table instead of fixed values, writes the directory cylinder and the bootstrap stepping rate into it, and leaves Index Register IY pointing at it, so that SYS0/SYS's reset copies the table to 4700H (its 4E6EH-4E7BH). BACKUP/CMD's DRIVER parameter asks for the interface type of each drive and loads a driver file to fill its drive code table (59B8H-5A2DH); the VTOS addendum names LOBO as such an interface.

4246H - Locate SYS0/SYS (Sector 1)

Bytes 16H and 17H of the directory record give the cylinder and the first granule of SYS0/SYS's first extent. Sector 1 works the granule into a sector with the sectors per granule from byte +08H of the drive code table (bits 4-0, the sectors of a granule less one) and sets up the alternate register set for the byte fetch at 4297H: Register D' the cylinder, Register E' the sector, Register Pair BC' the buffer pointer.

4260H - Load Record Loop (Sector 1)

The same loader as sector 0's at 4242H, fetching through 4297H: type 01H records are load blocks (the length counts the two address bytes, so 0 or 1 gives 254 or 255 data bytes), type 02H is the transfer record, every other type is skipped by its length. Register Pair HL is the load address and Register B the bytes left in the record.

428BH - Transfer Record: Start SYS0/SYS (Sector 1)

The transfer record gives the address SYS0/SYS starts at (4E00H on this diskette). The jump leaves interrupts off and Index Register IY pointing at the drive code table this sector was entered with, which SYS0/SYS's reset copies to 4700H.

4297H - Fetch the Next Byte of SYS0/SYS (Sector 1)

Returns the next byte of the file in Register A, working in the alternate register set (Register Pair BC' the buffer pointer, Register Pair DE' the next cylinder and sector). When the buffer at 5100H-51FFH is used up, the next sector is read through the driver; the sectors of a cylinder come from the operand at 42A5H that 422DH stored.

42ADH - Display an Error and Stop (Sector 1)

Entered at 42ADH when the driver cannot read a sector and at 42B0H with Register Pair HL pointing to NO SYSTEM. The text is displayed, the ROM waits for a line from the keyboard and the computer stops until RESET.

42B7H - Display a Text (Sector 1)

Displays the text at Register Pair HL through the ROM up to the end byte 03H (not displayed). Register Pair HL is unchanged on return.

42C5H - Read a Sector Through the Driver

Reads cylinder Register D, sector Register E of drive 0 into the buffer at Register Pair BC by calling the driver through the drive code table at Index Register IY. The driver is given the buffer in Register Pair HL, the function in Register B and the drive in Register C; it returns the Z FLAG set when the sector was read.

42D3H - Multiply Register A by Register E

An 8 bit multiply by shifts and adds: Register A returns the low 8 bits of Register A x Register E. Register D is left holding the old Register A and Register E is shifted to 00H; Register Pair BC is kept.

Sector 2: The System Information Sector

Sector 2 is shown at 4200H-42FFH, where SYS0/SYS's reset reads it at every boot (its 4F6BH-4F73H read cylinder 0, sector 2 of drive 0 over the boot sector). The reset checks bytes C0H-FFH (4F45H-4F4EH), takes byte 01H as an instruction (4F76H-4F79H), shows bytes C0H-C7H on the screen (4F7CH-4F85H) and copies the drive code tables of drives 1-7 (4F87H-4F90H). FORMAT/CMD writes the sector as 40H and 255 bytes of 00H (62E5H-62F5H); BACKUP/CMD copies it from the source diskette to the destination (660CH-6627H) and, with its DRIVER parameter, writes the drive code tables into it (5287H-52B2H); SYSTEM (SYSGEN) in SYS6/SYS writes byte 01H.

Sector 3: A Copy of a Granule Allocation Table

Sector 3 has the layout of the granule allocation table, the first sector of the directory cylinder; it is shown at 4200H-42FFH, the buffer where SYS2/SYS and SYS6/SYS read that table. FORMAT/CMD writes its new table here as well (62CDH-62E2H write its buffer at 6900H to cylinder 0, sector 3 before 6306H-631DH fill in bytes CCH and CDH and write the table to the directory cylinder). SYS0/SYS does not read this sector: it reads the table on the directory cylinder (4F9AH-4FA2H). The copy on this diskette is not this diskette's table: it holds 35 cylinders, the name VTOS40:F and the date 08/21/80, where the table on the directory cylinder (sector 0 of DIR/SYS) holds 42 cylinders, the name VTOS4.02 and the date 00/00/00.