TRS-80 DOS - NEWDOS/80 v2.0 for the Model I - SYS12/SYS Disassembled

SYS12/SYS

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NEWDOS/80 v2.0 SYS12/SYS Disassembly - BASIC REF (Model I)

SYS12/SYS is the BASIC command REF, a cross reference of line numbers, variable names and text in the program. It loads into the overlay area at 4D00H-51D2H (1,235 bytes in five load records, transfer address 4D00H) and is called through RST 28H by BASIC's direct statement code in SYS18 (555DH), with BASIC's end of statement (6445H) as the return address. Because it stays in the overlay area until another overlay is loaded, a plain REF can continue the last search. The work areas 51D3H-51EBH lie after the file.

The page shows every byte of the file once, in address order. Calls into BASIC/CMD (5700H up) are linked only where the BASIC page has a row with the right bytes at that address.

REF Parameters (from the code)

Variables and Patched Code

4D00H - REF: Read the Parameter

SYS12 is called through RST 28H by BASIC's direct statement code (SYS18 555DH) with Register Pair HL = the text after REF and the return address 6445H (BASIC's end of statement) on the stack. It stays in the overlay area afterwards, so a plain REF can go on from where the last one stopped as long as no other overlay has been loaded. Forms, from the code: REF (the next line holding the last item, listed), REF @n (the same from line n on), REF name or REF number (every line that uses it), REF "text or REF =text (every line holding the text), REF * / REF $ (every line number and variable name of the program, in order; *name / $name start at that name). A $ right after REF also sends the output to the printer.

4DB0H - The Full Report: the Next Item

Each round of REF * or REF $ first scans the whole program for the smallest line number or name above the last one reported (mode 0), keeping the addresses of up to 32 lines that use it in the DOS sector buffer at 4200H; then it reports that item (mode 1). Line numbers come before names. Holding BREAK pauses between items (ENTER goes on), the up arrow key stops.

4DFCH - End of a Line

Reached through the JP at 4F6DH at the end of each line, with Register Pair DE at the line's 00H end. If the line had a match (4F6DH was made a JP), its references are reported (4E80H); otherwise the scan goes on with the next line.

4E80H - Report a Line

Prints the line's number once for every kind of use found in the line: the name alone, with $ % ! # (the type characters at 51B2H-51B5H), and each of these as an array (followed by an opening parenthesis, shown with ( instead of /), with the count when it is used more than once. A line-number item or a text search has only the first counter. Register Pair DE is the text pointer at the line's end.

4F07H - Plain REF and REF @n

Finds the first line after the last one shown (or from line n on) that holds the remembered item, in mode 3, and lists it.

4F46H - Scan the Text

Register Pair DE walks through the program's text. Strings, REM and ' comments, DATA, constants after & and decimal fractions are skipped; line numbers (digit strings) and variable names are collected and compared with the item. A match counts in the counters at 51D1H-51DAH and turns the JP Z,4DFCH at 4F6DH into JP 4DFCH, so the line is reported at its end.

5101H - Remember the Line

Adds this line's address to the list at 4200H-423FH (32 lines) unless the list is full or the line is already its last entry.

512FH - Text Search

Reached through JP 512FH at 4F7CH with Register A = the current byte of the line. Compares the bytes from there with the search text at 51DBH until its 00H end; a match counts in 51D1H and the scan goes on after it.

514BH - Small Helpers

514BH: the next byte of the line in Register A, skipping spaces, 09H and 0AH. 515AH: CARRY if Register A is not a letter or digit; 5161H: CARRY if it is not a letter. 5168H/516EH: store the best item's first two characters (Register Pair BC) and length byte, and empty the line list; 5171H: set 5021H for the kind of item.

5189H - Compare Characters

Compares Register C characters at 51CCH with those at Index Register IY. The Z FLAG is set if they are all equal (or Register C is 0).

519BH - Position the Output

519EH prints a space; if the column is 53 or more it starts a new line, and at the start of a line it indents to column 7 (past the item's name).

51B2H - Type Characters and Work Areas

The rest of the file: the type characters, the TEXT END message and the work areas (all zero in the file). After the file (not loaded): 51D3H-51DAH the remaining counters and 51DBH-51EBH the search text.