DOSPLUS 3.5 for the Model III - DISKDUMP/CMD Disassembled

DISKDUMP/CMD

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DOSPLUS 3.5 DISKDUMP/CMD Disassembly - The File Sector Display and Editor (Model III)

DISKDUMP/CMD shows and changes a file sector by sector, run from the DOS command level as DISKDUMP filespec; its heading reads DISKDUMP - DOSPLUS File dump/edit - 3.50. It is a load module of 4 blocks, 938 bytes, at 5200H-55A9H with the transfer address 5200H, after the two comment records (the name DISKDUMP and the copyright notice). DISKDUMP/CMD is on the Model III master only.

The file is opened with 256 byte records, so a record is a sector of the file. DISKDUMP writes the whole screen straight into the video RAM: 16 rows of 16 bytes, each row with its offset, eight groups of two bytes in hexadecimal and the 16 characters, or the bytes as characters alone after CLEAR. The four left columns hold the drive number (top row) and the low byte of the sector number (second row), and, in the modify mode, the offset of the cursor (fourth row) and the byte under it (fifth row).

The Parts of DISKDUMP/CMD

How DISKDUMP Works

A file specification on the command line is opened at once; without one DISKDUMP shows its heading and an asterisk and takes up to 63 characters, ENTER alone asking again and BREAK returning to the DOS (520DH). A device is Illegal filespec. After OPEN@ the sector of the file control block's next record number is read by POSN@ and READ@, and BKSP@ sets the file back on it, so FCB+0AH always names the sector shown (524AH). Each command key ends by showing the sector again (5265H): the semicolon the next sector, the plus sign the last (the ending record number, less one when the end of file byte is 00H), the equal sign the first, the minus sign the previous; G takes a hexadecimal number. A sector before the first or past the last is not shown; the key is ignored (5307H). BREAK clears the screen and asks for another file without closing the one open (526CH).

Numbers are typed blind: hexadecimal digits are shifted into Register Pair BC until a space, ENTER or another control key, and other keys are ignored (544BH). F finds a byte, typed as a number in the hexadecimal mode or as one key in the character mode, and flashes two graphics blocks on every place it occurs until a key is pressed (52A0H). P reads the file from its start as a load module by GET@, passing each block byte by byte until the byte that loads at the address; the transfer record first is Invalid data provided, a record type of 20H or more Attempted to use non program file as a program (5323H). The byte found is the next one in the buffer, and the modify mode starts on it.

In the modify mode the cursor is two graphics blocks over the byte (5377H). The four arrows, SHIFT with the left and right arrows (the start and end of the row) and SHIFT with the up arrow (the first byte) move it through the table at 5535H; a digit pair (hexadecimal mode) or any other key (character mode) changes the byte and moves on; @ xxyy fills yy bytes with xx from the cursor, @ xx fills xx bytes with 00H, a count of 00H fills to the end (53F0H). ENTER writes the sector by WRITE@ and BKSP@; BREAK reads it again, so the changes are lost (5402H). Errors clear the screen, show the message, and go back to the asterisk prompt (5524H).

Differences from the Manual

The manual's table gives the colon for the next sector, its text the semicolon; the bytes take the semicolon only. Its text says F is used by typing G; F is the key. It does not say that the value to find is one key without ENTER in the character mode, that a number is ended by a space as well as ENTER and is not shown while it is typed, that the fill count 00H (and @ alone) fills to the end of the sector, that SHIFT with the arrows moves to the start or end of a row or of the sector, or how to leave DISKDUMP: BREAK at the command keys, then BREAK at the prompt. The sector number on the screen is its low byte only. When P does not find the address, the message is the DOS error Invalid data provided, then the prompt.

5200H - DISKDUMP

DISKDUMP/CMD's transfer address, entered from the DOS command level with Register Pair HL at the command line after the word DISKDUMP: DISKDUMP filespec. DISKDUMP opens the file and shows one sector (a 256 byte record) at a time on the whole screen, in hexadecimal with the characters beside it or as characters alone, and takes one key commands: the next, previous, first and last sector, G to go to a sector, F to find a byte, P to find the byte that loads at an address, M to change the sector and write it back. BREAK at the command level asks for another file; BREAK at the prompt returns to the DOS.

520DH - The Asterisk Prompt

Shows the heading and an asterisk and takes up to 63 characters as the file specification. BREAK returns to the DOS; ENTER alone asks again. 55A6H is set so that the next display clears the four left columns.

5235H - Open the File

A device (@) is the error 2CH. OPEN@ with 256 byte records and the buffer 5600H, then the sector named by the file control block's next record number (FCB+0AH) is read and shown.

524AH - Read and Show a Sector

Register Pair BC = the sector: POSN@, READ@ and BKSP@, so the next record number stays on the sector shown; then the display and the command keys.

5265H - The Command Keys

One key: BREAK another file, ; the next sector, + the last, = the first, minus the previous, F find, G go to, M modify, P find a load address, CLEAR the other display mode; any other key shows the sector again.

52A0H - F, Find a Byte

In the hexadecimal mode a number ended by a space or ENTER (its low byte counts); in the character mode one key. Every byte of the sector equal to it flashes as two graphics blocks (8FH) on its place, about 512 keyboard scans on and off, until a key is pressed; the key is not kept.

52E5H - The Last, the First and the Previous Sector

+ takes the ending record number (FCB+0CH), less 1 when the end of file byte (FCB+08H) is 00H; = sector 0; minus the sector before the one shown, nothing at sector 0.

5307H - The Next Sector and G

; the sector after the one shown; G a hexadecimal number. Either is shown only when it is not past the last sector (the ending record number, less 1 when the end of file byte is 00H).

5323H - P, Find the Byte of a Load Address

A hexadecimal address; the file is read from its start as a load module by GET@: type 01H blocks are passed byte by byte until the byte that loads at the address, other records skipped; type 02H (the transfer address) first is the error 2DH, a type of 20H or more the error 22H. The byte found starts the modify mode on its place.

5377H - M, the Modify Mode

Register Pair DE = the cursor in the buffer. Shows the sector, the offset of the cursor at 3CC0H and its byte at 3D00H, and the cursor as two graphics blocks; each key returns through 537AH to show it again. The arrows move the cursor (table 5535H), @ fills, CLEAR changes the mode, ENTER writes the sector, BREAK reads it again; any other key is data: a character, or two hexadecimal digits.

53F0H - @, Fill

A hexadecimal number xxyy: Register B = xx the byte, Register C = yy the count (@ xx alone fills xx bytes with 00H); a count of 00H fills to the end of the sector. The cursor does not move.

5402H - CLEAR, ENTER and BREAK

5402H changes the display mode (5405H changes the byte at Register Pair HL, also the flash phase). ENTER writes the buffer as the sector of the next record number by WRITE@, then BKSP@ keeps it there. BREAK reads the sector again, so the changes are lost.

541EH - The Screen Address of a Byte

Register E = the offset of a byte in the sector (row in bits 7-4, column in bits 3-0): returns Register Pair HL = the screen address of its first digit, 3C00H + 64 x row + 8 + 2 x column + column / 2. Register Pair BC is saved.

543AH - A Hexadecimal Digit

Register A = a character: returns the NC FLAG and Register A = its value 0-15 for 0-9 and A-F, else the C FLAG; Register B = the character.

544BH - A Hexadecimal Number

Takes keys until a space, ENTER, BREAK or any other key below 21H; each hexadecimal digit is shifted into Register Pair BC from the right, other keys are ignored. Nothing is shown.

5466H - Show the Sector

Writes the whole screen straight into the video RAM, in the second register set (EXX), so the main registers are kept. Each of 16 rows: four columns (spaces only on the first display after the prompt), the offset, a colon, then eight groups of two bytes, each a space and four hexadecimal digits (or a space, a character, a space, a character), a space and the 16 characters (spaces in the character mode). Then the drive (FCB+06H) at 3C00H and the low byte of the sector (FCB+0AH) at 3C40H; 55A6H is cleared.

54EDH - A Character, Two Hexadecimal Digits, a Place

54EDH puts the byte at Register Pair HL on the screen as a character, a period for a byte below 20H or from C0H on; 54FAH puts Register A as two hexadecimal digits (Register C changed); 550BH puts Register A at Register Pair DE and moves on.

550EH - A Key

Waits for a key by the ROM's 0049H; a lower case letter becomes upper case (5513H, also entered on its own). Register Pair DE is saved.

551CH - The Errors

551CH the error 2CH, Illegal filespec; 551FH, inside the LD BC at 551EH, 22H, Attempted to use non program file as a program; 5522H, inside the LD BC at 5521H, 2DH, Invalid data provided; 5524H any error code in Register A. The screen is cleared when it shows a sector, the error is posted with bits 7 and 6 set (ERROR@ returns, no frame), and DISKDUMP goes back to the asterisk prompt.

5535H - The Data

The table of the cursor keys of the modify mode (5535H, three bytes an entry: the key, the mask ANDed with the offset, the addend), the heading and the variables. The file control block 55AAH-55C9H and the buffer 5600H are not loaded.