LDOS 5.3.1 for the Model III - BASIC/OV2 Disassembled

BASIC/OV2

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LDOS 5.3.1 BASIC/OV2 Disassembly - CMD"X", the Cross Reference of a BASIC Program (Model III)

BASIC/OV2 lists the variables of the BASIC program in memory and the line numbers it refers to, each with the lines where it is met: CMD"X [devspec or filespec] [-V or -L or =entry] [<title>]". BASIC/CMD runs it with @RUN when CMD is given an X and a space (its 547CH); the file loads over BASIC/CMD at 5200H, and when it is done it runs BASIC/CMD again with an asterisk, back to the program in memory with its variables (568DH).

The file is a load module of 1647 bytes: a copyright record, the code and data 5200H-581BH in seven load blocks, and the transfer address 5200H.

The Parameters

How It Works

About This Page

5200H - The Start: the Banner and the Output

The transfer address. BASIC/CMD runs this file with @RUN for CMD"X text" (its 547CH): the file loads over BASIC/CMD at 5200H and is entered with Register Pair HL at the text after the X, which BASIC/CMD copied to 4226H with a 0DH after it. The banner is shown and the text folded to upper case. When the text starts with an asterisk or a character from A up, it is the device or file for the listing: @FSPEC reads it into the control block at 5847H, a file gets the extension TXT when it has none, and @INIT opens it with the buffer 5900H. 5793H keeps the first byte of the specification (2AH for a device).

5246H - The Options, the Title and the Heading

A new line on the video, then the output of 55F6H is pointed at the control block 5847H when a device or file was given (it is the video's, 401DH, from 55E4H otherwise). 5570H reads -V, -L, =name and the title in angle brackets. The title and the heading are sent, 581CH (the last entry listed) is set to seven spaces, below every entry, and the list of line numbers is emptied.

5273H - One Pass: the Next Entry

Each pass reads every line of the program from 40A4H and takes each entry of it (a line number after a reference token, or a variable name) as seven characters at 582CH (538AH). With -V only names are taken, with -L only line numbers. The pass looks for the lowest entry above the last one listed (581CH): a new lowest entry goes into 5824H and the list of line numbers starts again; each line where that entry is met is added to the list (552FH). With =name (5844H) only the named entry is taken. 5840H is set when an entry is found.

52EFH - Listing One Entry

The list is ended with 0000H. With no entry found, the listing ends (5354H). Else the entry becomes the last one listed (581CH), a rule line is drawn (a new page first after 50 lines), the reference column is sent as a colon, the first five characters of the entry and a colon, and then the line numbers of the list, a line number once even when the entry is met more than once in a line, eight to an output line; a further output line starts with an empty reference column.

534DH - The End of the Listing

After an entry: the next pass, unless =name was given. At the end: a rule line and a form feed, then the file is closed and BASIC/CMD run again.

5360H - Comparing Two Entries

Compares the text at Register Pair HL with the text at Register Pair DE up to the 00H of either: the Z FLAG when they are the same, else the CARRY FLAG when the text at Register Pair DE is the lower. Both are kept.

5374H - Copying an Entry

Copies the bytes at Register Pair HL to Register Pair DE up to the 00H, which is not copied. Register Pair BC is counted down by the LDI.

537BH - An Empty Entry

537BH fills the seven bytes at Register Pair HL with spaces, 537DH with the byte in Register A, and puts 00H in the eighth. Register Pair HL and Register Pair BC are kept.

538AH - The Scanner: the Next Entry of a Line

Reads the text of the line from Index Register IY +04H on and puts the next entry in the field at Register Pair HL, leaving Index Register IY after it for the next call; nothing is stored at the end of the line. Passed over: a string in quotes, the rest of a line after REM (93H), DATA (88H) up to a colon, a constant after an ampersand, a number (and its letters, such as an exponent), and every other byte. After THEN, ELSE, RESUME, GOTO, GOSUB, RUN or ERL each line number is taken (5414H); 5842H tells the next call that more line numbers may follow. A name is its first two characters, then its type character (%, $, # or !) and a ( for an array; the other characters of a long name are passed over.

5406H - A String in Quotes

Passes over the text up to the closing quote, then goes on after it (5393H); the end of the line returns with nothing stored.

5414H - A Line Number after a Token

Spaces, commas, the = token (D5H) and the digit 0 are passed over, so a line number is taken without its leading zeros and 0 alone is not taken. Up to five digits are stored and the number is moved right with zeros in front, five digits wide, so that line numbers sort by their value and before the names. 5842H is set to 01H so the next call looks for a further number of the list. Any other byte goes back to the scanner (5395H).

5472H - Classing a Byte

5472H stores the byte at Index Register IY +04H at Register Pair HL, steps both and goes on to 5479H. 5479H classes the byte at Index Register IY +04H into Register C and Register A: 01H a digit, 02H a letter A-Z, 03H a type character (%, $, # or !), 04H an open parenthesis, 00H any other byte (a token, a lower case letter, a sign); the Z FLAG for 00H.

54AEH - Ending an Output Line, a New Page

54AEH ends the output line with 0DH, but from 55 lines on (5794H) it starts a new page. 54B9H starts a new page from 50 lines on: a rule line, a form feed and the title (5721H), the heading (5760H), then the rule line of 54CFH; below 50 lines only that rule line. 54CFH sends 0DH, a plus sign, five minus signs, a plus sign and 0DH; Index Register IX and Index Register IY are kept.

54F6H - A Field of the Output Line

54F6H starts an output line (the count of fields at 5841H to 00H). 54FAH clears the field at 5834H to seven spaces and counts it; the first field of a line, the reference column, gets a colon at each end (5834H and 583AH). Returns Register A = the count of fields and Register Pair HL = 5834H.

5515H - The List of Line Numbers

5515H points 583CH at the start of the list, 5 bytes above the end of the arrays (40FDH). 5521H ends the list with 0000H, clears the last number read (583EH) and points 583CH back at the start. 552FH adds the number of the line at Index Register IX to the list; when the list would go above the bottom of string space (40A0H), Out of memory is shown on the video and the listing ends. The list may grow up to the address in 40A0H, and the stack this file runs on lies below that address (the ROM sets the Stack Pointer from 40A0H at 1B9AH).

5555H - The Next Line Number of the List

Reads the next number from the list at 583CH into Register Pair HL, passing over a number equal to the one before (an entry met twice in a line); returns the NZ FLAG with the number, or the Z FLAG at the end mark 0000H. A program line numbered 0 would end the list there, its number being the same as the end mark.

5570H - The Options

Register Pair HL points at the text after the device or file. A minus sign stores the letter after it at 5843H (V for the names only, L for the line numbers only; any other letter keeps both). An equals sign takes the name or line number after it into 5824H the way the scanner takes an entry, and sets 5844H; a byte after it that is neither a digit nor a letter names nothing. Only the first of a minus sign and an equals sign is read. Then the text from the first < is the title, up to > or 0DH, at most 58 characters, copied to 5722H with 0DH and 00H after it; the character after the < is copied before any test.

55E4H - Sending a Text

55E4H points the output at the video's control block (401DH), 55EEH at the printer's (4025H; no code calls 55EEH), then 55F6H sends the text at Register Pair HL, up to its 00H, a byte at a time to the device or file whose control block address is the operand at 55FBH, with ROM 001BH. Each 0DH counts a line in 5794H, a 0CH starts the count again. After each byte the keyboard task is run, BREAK ends the listing (567EH) and SHIFT @ pauses it until a key other than SHIFT @ (BREAK then ends it). An error on a device is not reported; an error on a file goes to 5656H. Register Pair DE, Register Pair HL and Index Register IY are kept.

564DH - A File or a Device

Tests bit 7 of the first byte of the control block at Register Pair DE, set for an open file: the NZ FLAG for a file, the Z FLAG for a device. Register A and Register Pair BC are kept.

5656H - An Error Writing the File

Meant to give the error with @ERROR and end the listing. The CALL 5648H takes 565BH as the saved Register Pair DE, the saved Register Pair DE as Register Pair HL and the saved Register Pair HL as Index Register IY, and returns to the address saved from Index Register IY at 55F6H, a place in the program text: 565BH is not reached.

5661H - The Keyboard Flags

Loads KFLAG$ (429FH) into Register A; when bit 0 (BREAK) or bit 1 (SHIFT @) is set, the keys waiting are read away with ROM 002BH. Returns the NZ FLAG for BREAK (bit 0). 5673H clears bits 0-2 of KFLAG$; Register Pair AF is kept.

567EH - BREAK

The keys are cleared, the output line is ended, Break is shown on the video, then the listing is closed and BASIC/CMD run again (568DH). The stack of 55F6H is left as it is, since nothing returns.

568DH - Back to BASIC

The listing file is closed (5710H), the name BASIC/CMD put in the control block at 4485H and 2AH (an asterisk) over the first byte of the command line at 4225H; @RUN loads BASIC/CMD and runs it with Register Pair HL at 4225H, and BASIC with an asterisk goes back to the program in memory with its variables.

56B0H - Upper Case

Turns the lower case letters (61H-7AH) of the text at Register Pair HL into upper case, up to the first control character; Register Pair AF and Register Pair HL are kept.

56C8H - A Line Number as Text

The ROM's LINPRT (0FAFH) without its display: Register Pair HL goes to the ROM's number buffer as text (at 4130H, a space for the sign first, ended by 00H). Returns Register Pair DE at the first digit (4131H); Register Pair HL is kept.

56D8H - The Next Line by Its Text

Moves Index Register IX past the link and the number (4 bytes) and past the text to the 00H after it: Index Register IX = the next line. Then 56E8H: the Z FLAG when the link of the line at Index Register IX is 0000H, the end mark of the program.

56F2H - Tokens Followed by Line Numbers

Returns the Z FLAG when Register A holds the token of THEN (CAH), ELSE (95H), RESUME (9FH), GOTO (8DH), GOSUB (91H), RUN (8EH) or ERL (C2H). The line numbers after RESTORE, LIST, EDIT and DELETE are not taken.

5708H - A DOS Error

From @FSPEC or @INIT: the error number in Register A is given to @ERROR with bit 6 (the short message) and bit 7 (return) set, then the listing ends.

5710H - Closing the Listing File

When 5793H holds a letter (41H or above), the output is a file: 5793H is cleared and the control block at 5847H closed with @CLOSE, its result not tested. The video (00H) and a device (2AH) are not closed. After an @INIT error the control block of a file not opened is given to @CLOSE too.

5721H - The Texts and the Work Cells

The title buffer with the form feed before it, the heading, the pieces of the rule line, the kind of output and the count of lines, the messages, the default extension and the banner. Every text sent by 55F6H ends with 00H; the banner is shown with @DSPLY and ends with 0DH. The cells from 581CH on, the control block at 5847H and the buffer at 5900H are not in the file.