DOSPLUS 3.5 for the Model I - BACKUP/CMD Disassembled

BACKUP/CMD

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DOSPLUS 3.5 BACKUP/CMD Disassembly - The Mirror Image Floppy Backup (Model I)

BACKUP/CMD is DOSPLUS's backup utility, run from the DOS command level as BACKUP [FROM] :sd [TO] :dd (DATE="string",USE="string"); its heading reads BACKUP - DOSPLUS Floppy backup - 3.51. It is a load module of 16 blocks, 2730 bytes, at 5200H-5CB3H with the transfer address 5200H, after the two comment records (the name BACKUP and the copyright notice) and a 07H record. The last four blocks are patches that load over bytes already loaded: three blocks put new code at 53B3H-53CBH (5396H), and one changes 59F7H from 30H to 31H, so that the heading shows 3.51 instead of 3.50 (59D3H). The page shows the bytes as they are after the patches. The Model I and Model III files have the same length and differ in 47 bytes: the addresses in SYS0/SYS (DATE$, LOW$, HMEM$, SFLAG1$, DCTTBL, DISKIO%, LOCDCT%, the multiply and divide and the routine that saves the registers) and the three prompts Insert ... disk, which end (ENTER) on the Model I and [ENTER] on the Model III; this page is the Model I one.

BACKUP copies a floppy diskette as a mirror image: every granule that the source's GAT shows as allocated, and not locked out, is copied to the same cylinder, granule and sector of the destination. It refuses a rigid drive (Drive NOT SUPPORTED!, 567AH) and a copy between an 8 inch and a 5 inch drive (Drives INCOMPATABLE!, as the file spells it, 56F6H). With the same drive as source and destination it prompts for each diskette in turn (540DH).

The Parts of BACKUP/CMD

How BACKUP Works

The buffer of sectors starts at 6100H, or at the page above LOW$ (4047H) when that is higher, and needs 3BFEH bytes below HMEM$ (4049H) for the image of a track (else No available memory!, 5200H). EVAL@ takes the drives and the two parameters from the command line (5237H); a drive not given is asked for with Source drivespec ? or Destination drivespec ?, whose reply is the drive's one or two character name in DCTTBL, without a colon (595AH). The date comes from DATE=, else from the system date as MM/DD/YY, else from the prompt Backup date ? (528DH).

BACKUP checks the source drive and notes whether its diskette is write protected, finds its density from sector 2 of cylinder 0 and reads its GAT (5636H). From then on both drives are given the source's geometry (581CH). A double density diskette whose cylinder 0 is in single density, as a Model I system diskette must be for the ROM to boot it, is noted (5CB5H): cylinder 0 is then read, written and formatted as one single density granule of 10 sectors (5993H).

The destination (56A4H) is formatted when it cannot be read; otherwise Diskette contains data, use or not ? is answered by USE= or at the prompt: N ends BACKUP, F formats all of it, Y or U use its format. A diskette that is used but has fewer cylinders, another density or another count of sides is formatted all the same; else only cylinder 0 (when its density differs) and the old directory cylinder (when the directory moves) are formatted, and the destination keeps its own lock outs and, when it has more cylinders than the source, its own count. Formatting (5743H, 54F8H) writes each track with an interleave of 2 in single density and 3 in double density, starting each track twice the interleave further on, with 256 bytes of 6CH in every sector; then every granule is read back from the last cylinder down, and one that cannot be read is locked out (GRAN FLAWED! Locked out!); a flawed boot granule ends BACKUP. The new GAT (57B7H) holds the granules to copy and the destination's lock outs; a granule to copy that is locked out on the destination ends BACKUP with BACKUP rejected! Too many FLAWS!. Its bytes +C0H-+FFH (the name, the master password, the AUTO command) are the source's, with the date of the backup, the version 35H and the count of cylinders.

The copy loop (5320H) reads the granules to copy, cylinder after cylinder, as long as another whole cylinder fits below HMEM$, writes that run to the destination with each sector's data address mark (DWRITA for a sector that was read with the directory's mark, 54D0H) and verifies it, then goes on with the next run. Last (5396H) the directory cylinder is read from the source, the modified flag of every directory entry is cleared, the cylinder is written back to the source unless it is write protected, and written to the destination with the new GAT in place of the source's. Both drives are then marked to be logged again, and Insert SYSTEM disk (ENTER) is flashed when drive 0 was one of them (5612H).

The 3.51 Patch

The 3.50 code at 53B3H-53CBH was LD HL,(5CBBH) / INC H / INC H / INC L / LD A,(IY+0EH) / SUB 02H / LD B,A, then RES 6,(HL) / LD A,20H / ADD A,L / LD L,A / JR NC / INC H / DJNZ, between two EXX: it cleared bit 6 of byte +01H in every 32 byte entry of the directory sectors. In an extended entry (FXDE, bit 7 of byte +00H set) byte +01H is not a flag byte but the link back to the file's first entry, so the old code could change that link. The patched code tests bit 7 of each entry's first byte and leaves the FXDEs alone (53BDH); it no longer keeps the registers by EXX, which the code after it does not need.

Findings

BACKUP does not test EVAL@'s return code (523BH): an unknown parameter or a value of the wrong type is passed over without a message. The entries USE and DATE take only a string in quotation marks (type 80H, 5C96H); EVAL@ stores the address of the string's text over the operands of two LD HL instructions, and BACKUP copies up to 8 characters from there (5891H). The verify pass reads every sector back but does not compare it with what was written (5483H). A read error in the verify pass, or while the source's directory cylinder is read, shows Destination disk WRITE ERROR! (53B0H); the messages I/O ERROR! routine aborted! and Destination disk READ ERROR! are never shown (55CCH). A read whose code is 06H or more counts as a good read: 06H is the record type of the directory's data address mark, and the code less 05H is kept as the sector's mark (54C3H). The flashing prompts wait for any key, not only ENTER, and are not shown while a DO file runs (58DAH). An answer of N at Diskette contains data ends BACKUP with EXIT@, not with an error.

Differences from the Manual

The manual (Utilities, BACKUP, pages 3-1 to 3-5, and the first backup in the getting started section) gives the parameters with USE=Y or U=Y; the bytes take only USE="Y" (any string, its first character the answer), and with USE=Y EVAL@ refuses the value and BACKUP prompts all the same. The manual lists Y, U, F and BREAK at Diskette contains data, and N in the getting started text; the bytes take N, F, U and Y, in either case. In a backup with one drive the manual says BACKUP may need the system disk from time to time; the bytes prompt for it only at the end, and only when drive 0 was the source or the destination. The manual's statements that only allocated granules are copied, that the destination may keep its format, that formatting is done as little as needed, that a granule locked out on the destination aborts the backup, and that the modified flags of both directories are cleared, agree with the bytes; that BACKUP does not de-allocate the extra cylinders of a destination that has more agrees too (5708H).

5200H - BACKUP

BACKUP/CMD's transfer address, entered from the DOS command level with Register Pair HL at the command line after the word BACKUP: BACKUP [FROM] :sd [TO] :dd (DATE="string",USE="string"). BACKUP shows its heading, sets its buffer above 6100H, takes the drives from the command line or from the prompts, takes the date, checks both drives, reads the source's GAT, prepares the destination (56A4H: format it, or use its format), then copies every allocated granule cylinder by cylinder, as many as fit in memory at a time (read, write, verify), and last copies the directory cylinder with the modified flags cleared. It ends with the prompt for the system disk and EXIT@, or with an error message and ABORT@.

528DH - The Date

The date written into the destination's GAT: the DATE string of the command line (its address stored over the operand at 528EH by EVAL@), else the system date DATE$ as MM/DD/YY, else the reply to the prompt Backup date ?. A month of 0 means that no date is set; ENTER alone at the prompt then gives 00/00/00.

52DBH - The Source Diskette

Checks the source drive (DCHECK), notes whether its diskette is write protected (5CB9H), reads its GAT into 5F00H and keeps its format (5636H): the format code in 5CB4H, the density flag of cylinder 0 in 5CB5H, and its ten drive control table bytes from DCT+0BH in 599DH-59A6H, the geometry used for both drives from now on.

5306H - The Destination Diskette

Checks the destination drive (DCHECK: not ready or write protected ends BACKUP), then prepares the destination diskette (56A4H): it is formatted, or its format is used, and its new GAT is made at 6000H.

5320H - The Copy Loop

One round copies a run of cylinders: from the next cylinder that has a granule to copy (5CB7H), the allocated granules are read into the buffer from (5CBBH) until no other cylinder would fit below HMEM$ (5CB6H is then the cylinder after the run), then written to the destination and verified there. A granule to copy is one allocated in the source's GAT and not locked out (580EH). When no cylinder is left, the directory cylinder is done at 5396H.

5396H - The Directory Cylinder

Reads the source's directory cylinder (its allocated granules) into the buffer, clears the modified flag of every directory entry there (the 3.51 patch, 53B3H-53CBH), writes it back to the source unless the source is write protected, puts the destination's new GAT (6000H) in place of the source's, and writes the cylinder to the destination. The patch blocks of the file load new code over 53B3H-53CBH: the 3.50 code (D9 2A BB 5C 24 24 2C FD 7E 0E D6 02 47 CB B6 3E 20 85 6F 30 F8 24 10 F5 D9) cleared bit 6 of byte +01H of every entry with the registers kept by EXX, also in an extended entry (FXDE), where byte +01H is the link back to the first entry; the new code skips the FXDEs.

540DH - The Prompt for a Disk

Only for a backup with one drive (5CBAH = 00H): shows the flashing prompt at Register Pair HL (58DAH) and waits for a key. With two drives it does nothing.

5415H - The Read Pass

Reads the granules to copy of cylinder Register D of drive Register C into the buffer at Register Pair HL, one page a sector, and stores a mark for each sector at Index Register IX (00H, or 01H for a sector with the directory's data address mark, from DISKIO%'s code 06H less 05H). It shows READING and the cylinder first. Returns the Z FLAG, or the NZ FLAG with Register A = the DISKIO% error.

5434H - Read a Granule

Reads the sectors of granule Register E of cylinder Register D into the buffer from Register Pair HL on (Register H stepped once a sector), storing each sector's mark at Index Register IX. Register Pair DE is kept.

544CH - The Write Pass

Writes the granules to copy of cylinder Register D to drive Register C from the buffer at Register Pair HL, each sector with the mark kept at Index Register IX in the read pass (54D0H: DWRITE, or DWRITA for the directory's data address mark). It shows WRITING and the cylinder first.

546BH - Write a Granule

Writes the sectors of granule Register E of cylinder Register D from the buffer at Register Pair HL, each with the mark at Index Register IX. Register Pair DE is kept.

5483H - The Verify Pass

Reads back the granules to copy of cylinder Register D of drive Register C (54A2H), each sector into the first page of the buffer: it checks that every sector can be read, but does not compare the data with what was written. It shows VERIFYING and the cylinder first.

54A2H - Verify a Granule

Reads every sector of granule Register E of cylinder Register D into the page at (5CBBH), the start of the buffer. Also used after formatting (5785H) to find the flawed granules. Register Pairs DE and HL are kept.

54B9H - The First Sector of a Granule

Register E = granule Register E times the sectors per granule (DCT+0FH); Register D is kept.

54C3H - Read a Sector

DREAD (function 03H) of sector Register E, cylinder Register D, drive Register C to Register Pair HL. Codes 01H-05H are errors (the NZ FLAG); 00H returns Register A = 00H with the Z FLAG; a code of 06H or more (06H: the record type of the directory's data address mark) returns the Z FLAG with Register A = the code less 05H, the sector's mark.

54D0H - Write a Sector

Writes sector Register E of cylinder Register D of drive Register C from Register Pair HL: DWRITE (function 05H) for the mark 00H in Register A, DWRITA (08H, with the directory's data address mark) for any other mark.

54DBH - Format a Cylinder

Formats cylinder Register D of drive Register C with the source's geometry: side 0, then side 1 for a diskette with two sides (Register E = the sectors per track, the first sector of side 1). It shows FORMATTING and the cylinder first.

54F8H - Make and Write a Track

Builds the image of one track in the buffer at (5CBBH) and writes it with DFORMT (function 09H). The sector order: the interleave Register A from 5847H (2 in single density, 3 in double) is laid out from a starting place that moves by twice the interleave on each track (the skew), into the table at 5D12H (place -> sector number). The image is made from the format description at Register Pair DE: pairs of a count and a byte (55C2H), with the ID fields (FEH, cylinder, side, sector, length code 01H, F7H for the CRC) and 256 data bytes of 6CH after FBH in each sector.

55C2H - Store a Group

Takes a count and a byte from Register Pair DE (a count of 00H is 256) and stores the byte that many times at Register Pair HL; both pointers are left after them.

55CCH - The Errors

Each entry points Register Pair HL to its message and goes to 5606H, which shows it, logs both drives again (with the prompt for the system disk when drive 0 was used) and leaves by ABORT@. 55E0H (I/O ERROR! routine aborted!) and 55FEH (Destination disk READ ERROR!) are reached by nothing: a read error in the verify pass and in the source's directory cylinder shows Destination disk WRITE ERROR!.

5612H - The End

Sets the log flag (bit 0 of DCT+03H) of the source and destination drives, so that the DOS reads their real geometry again on the next access, then flashes the prompt at Register Pair HL (Insert SYSTEM disk) and waits for a key when either drive was drive 0; else it only clears the line.

5636H - Read a GAT and Find the Format

Finds the density of drive Register C (567AH), then reads its GAT (SREAD of directory sector 0, which logs the diskette) into Register Pair HL. 563AH, entered after 567AH, is the same without the density test. Sets DCT+13H from the GAT's cylinder count (+CCH plus 35) and returns the Z FLAG with Register C = the format code (bit 0 double density, bit 1 8 inch, bit 2 two sides), Register B = 80H when SREAD had to change the density (cylinder 0 is in another density than the directory, as on a double density Model I system diskette) else 00H, and Register Pair HL = DCT+0BH; or the NZ FLAG and the error code in Register A.

567AH - Find the Density

LOCDCT% for drive Register C; a rigid drive (bit 5 of DCT+03H) ends BACKUP with Drive NOT SUPPORTED!. Sets the log flag, then verifies sector 2 of cylinder 0 (DVERF) in single density, and if that fails in double density: the density bit 7 of DCT+0BH is left as the one that worked. Returns the Z FLAG when one of them worked.

56A4H - Prepare the Destination

Makes 5CB8H, the GAT byte of an empty cylinder (a bit set for each granule a cylinder does not have). Then a destination that cannot be read in either density is formatted (5743H). Otherwise the answer to Diskette contains data, use or not ? is taken from USE= (its first character) or from the prompt: N ends BACKUP, F formats, Y or U uses the diskette (56EAH). A destination whose GAT cannot be read, or with fewer cylinders, another density or another count of sides is formatted; one of another drive size (8 inch or not) ends BACKUP with Drives INCOMPATABLE!. Else only cylinder 0 is formatted when its density flag differs, and the old directory cylinder when the directory is elsewhere; the destination's own lock outs are kept. The new GAT is made at 6000H (57B7H).

5743H - Format the Destination

Makes an empty GAT at 6000H (FFH, with the empty cylinder byte 5CB8H for each cylinder in the allocation and lock out tables), formats every cylinder with the source's geometry, then verifies each granule from the last cylinder down (54A2H): a granule that cannot be read is shown as GRAN FLAWED! Locked out! and set in both tables. A flawed granule 0 of cylinder 0 (the boot) ends BACKUP.

57B7H - Make the New GAT

For every cylinder, the granules to copy (the source's allocation XOR its lock out table) must not be locked out on the destination (else BACKUP rejected! Too many FLAWS!); the destination's allocation byte becomes those granules together with its own lock outs. The bytes +C0H-+FFH come from the source's GAT (the diskette name, date and AUTO command among them), then the date is put in +D8H-+DFH, the version 35H in +CBH and the count of cylinders less 35 in +CCH.

5800H - The Next Cylinder to Copy

5800H steps Register D on first. From cylinder Register D on, finds the first cylinder with a granule to copy (580EH): returns the Z FLAG with Register D that cylinder, or the NZ FLAG (and the C FLAG) when Register D is past the last cylinder.

580EH - The Granules to Copy

Register A = the source's allocation byte of cylinder Register D (5F00H on) XOR its lock out byte (5F60H on): a bit set for each granule allocated and not locked out; the Z FLAG when there is none.

581CH - Give a Drive the Source's Geometry

LOCDCT% for drive Register C, then copies the source's nine bytes DCT+0BH-DCT+13H (599DH) into the drive's DCT+0BH on, or for cylinder 0 of a diskette whose cylinder 0 is in the other density the single density values at 5993H. Every register but Index Register IY is kept.

5847H - The Format of a Track

Returns Register A = the interleave, Register Pair DE = the format description and Register Pair HL = 8001H (01H the sector length code, 80H the count of the data loop): for cylinder 0 of a diskette whose cylinder 0 is in the other density, 02H and the single density description 59A7H; else the entry of the format code's bits 0-1 in the table at 5987H. Index Register IX is changed.

5878H - A Prompt and Its Reply

Shows the text at Register Pair HL up to its 03H, then as many spaces as the byte after the 03H gives, and takes a reply of up to Register B characters into 5CC6H with the ROM's line input (0040H). BREAK ends BACKUP at 5404H. Returns Register Pair HL = 5CC6H and Register A = the first character of the reply.

5891H - Copy Up to 8 Characters

Copies the text at Register Pair HL to 5CC6H, at most 8 characters, ending at a control character or at a double or single quotation mark (the end of a string given to DATE or USE), and fills the rest of the 8 with spaces. Returns Register Pair HL = 5CC6H.

58B5H - Show a Text

Shows the characters at Register Pair HL with the ROM's 0033H until the 03H that ends the text. Returns Register Pair HL past the 03H; Register Pair DE is kept.

58C1H - Show a Pass and the Cylinder

Shows the text of the pass at Register Pair HL (a carriage return and a cursor up start it, so the line is written over), then Cylinder and the cylinder Register D in two decimal digits (stored over the 00 at 5ACBH).

58D1H - Upper Case

Makes a lower case letter in Register A upper case; any other character is left as it is.

58DAH - Flash a Prompt

Flashes the text at Register Pair HL (Insert SOURCE disk and the like) until a key is pressed: 500 scans of the keyboard with the text shown, 500 with the line cleared, and so on. The line is cleared at the end. Nothing is done while a DO file runs (bit 5 of SFLAG1$), so a DO file can run BACKUP. The registers are kept (4525H).

5905H - Clear the Line

Shows a carriage return, a cursor up (1BH) and an erase to the end of the line (1EH): the line of the cursor is cleared and the cursor is at its start. Sets the flash state 5917H to 1EH.

5918H - Two Decimal Digits

Stores Register A (0-99) at Register Pair HL as two ASCII digits, the tens first; Register Pair HL is left after them and Register Pair DE is kept.

592AH - Set, Reset or Test Bit E

592AH sets bit Register E (0-7) of Register A, 593AH resets it, and 594AH tests it (the Z FLAG when it is clear): each stores the bit number into the second byte of its SET, RES or BIT instruction (5938H, 5948H, 5958H) and then runs it.

595AH - A Drive Name

Looks up the two characters at Register Pair HL (made upper case) among the names of the eight entries of DCTTBL (43ABH: a DCT address and a two character name): returns the Z FLAG and Register A = the drive number, or the NZ FLAG when no name matches. A reply of one character matches a name whose second character is a space, since the reply is padded with spaces.

5987H - The Formats

Data: the table of 5847H, one entry for each value of the format code's bits 0-1 (an interleave, then the address of a format description); the single density values of cylinder 0 for 581CH; the work area of the source's drive control table bytes; and the two format descriptions, pairs of a count and a byte for 55C2H (a count of 00H is 256).

59D3H - The Texts

Data: the heading, the prompts (each followed by its 03H and the count of spaces 5878H shows after it), the texts of the passes and the messages, each ended by 03H for 58B5H. The heading's last digit 59F7H is the patch byte that makes the version 3.51 (31H; 3.50 as first loaded).

5C96H - The Evaluation Block and the Variables

Data: the evaluation block of EVAL@ (5237H) and its parameter block (9 byte entries: a six character name, the address for the value, the type byte 80H for a string; the address stored is that of the string's text, here the operands of two LD HL instructions), then the drives. 5CB4H on is not loaded.