PASS0: setup and dialog

What it does

PASS0 is the first of the three programs that make up the compiler. It talks to the user, prepares DOS and memory for the compilation, and leaves everything the later passes need in the COMMON block it defines. It does no compiling itself. Its work, in order:

1. Changes six bytes of DOS so that the tokenized Applesoft source can be read through DOS's text-file READ.
2. Asks for the source file, the object file, the memory layout and the compiler options, checking each answer.
3. Builds the tables the code generator uses (the call table and the operator table) and loads a small machine-language helper into page 3.
4. Sets up the symbol table's chain heads, creates the object file, empties the line number file, and runs PASS1 with BRUN.

Everything PASS0 hands on lives in the COMMON block, which PASS0 declares with REM! DEFCOMMON and PASS1 and PASS2 declare with REM! USECOMMON in exactly the same order. REM! CLEAR CHAIN just before the BRUN sets up string storage so the COMMON strings survive the chain.

Flow

1. DOS and memory (line 100). NOMONC,I,O stops DOS echoing commands, MAXFILES3 gives three file buffers (source, object, line number file), and HIMEM: 38400 keeps strings below the DOS buffers at $9600. The six DOS patches are read from the DATA statement at line 170; each original byte is saved in DP() for the restore at the end of the compilation (line 180).
2. Fixed values (line 230). The library routine that resets the stack (EL), the library length (LM, 4013), the default library address (DH, $0803), the DOS command strings, and the boot drive: DOS's last-used slot and drive ($AA6A, $AA68) become SQ, DO and DN$; the slot also gives the drive motor switches MX and MN, and $AA52 is DOS's output state byte DW.
3. Source file (line 400). The title, then SOURCE FILE?. The answer is split into a name and a slot/drive part (line 2800) and checked by running APPEND on it, which fails with FILE NOT FOUND if the file is missing (line 600, line 3480). The file is closed again at line 700.
4. Object file (line 700). The default is the source name, cut to 26 characters, plus .OBJ, on the source file's drive. Any other answer is split the same way (line 900).
5. Memory layout (line 960). The default puts the library at $0803, the program right after it and the variables after the program. Otherwise three addresses are asked for: the library (line 1060), the program (a number, HGR1 = $4000 or HGR2 = $6000, line 1220) and the variables (line 1420); each is checked by the address routine (line 3220). The layout is shown and confirmed (line 1500).
6. Options (line 1630). The default is listing, pause on errors, integer arithmetic and integer constants on, RESUME/debug code off; otherwise each is asked in turn. Return alone accepts the default.
7. Tables (line 2060, line 2160, line 2240). The call table and the operator table are read from DATA into the symbol table array SC() at 150 and 300, where PASS1 picks them up; the 89 bytes of machine code are POKEd into $0377-$03CF and the USR vector at $0A is pointed at $0377.
8. Symbol table (line 2310). 32 hash chains each for variables, constants and line numbers, and the addresses of the page-3 routines.
9. Files and chain (line 2440). A one-byte BSAVE creates the object file as a binary file; the line number file is opened, deleted and opened again so it is empty; then BRUN PASS1 on the boot drive.

How the dialog is built

The questions do not call their checks with GOSUB. A question stores a number in QH, saying which question is being answered, and jumps (GOTO) to a shared check. The check ends with ON QH GOTO, which returns to the code after that question (line 3410 for addresses, line 3590 for files). After every check, BZ = 1 means the answer was refused and the question is asked again.

• The yes/no routine (line 2570) returns XB = 1 for Y, 0 for N, 2 for Return alone; anything else rings the bell and sets BZ.
• The line reader (line 2700) calls the ROM's INLIN and copies the line from $0200 into LU$; Control-C stops the program.
• A file answer that starts with Control-D is a DOS command (CATALOG, for example): it is executed and the question is asked again (line 3450).
• DOS errors are caught with ONERR; error 6 is reported as FILE NOT FOUND, anything else as DISK ERROR. After a caught error the library routine at $0947 (EL) resets the stack, because Applesoft's error handling leaves it as it was when the error happened.
• The address routine (line 3220) accepts a decimal number (converted with VAL under ONERR) or hexadecimal after $, from 0 to 49151 ($BFFF).

Subroutines

Stored addresses

Integer variables hold -32767 to 32767, so an address is usually kept as address-32767 and turned back with +32767 (FN SA and FN RA in this pass; the other passes have the same pair). Addresses that are POKEd or PEEKed are kept as signed words instead, address-65536 above 32767 (FN SW): that is how DP(I,0), MX, MN and DW are stored.

The COMMON block

PASS0 defines it at the start of program space ($17B0, right after the library) and moves the program start to $41F2. Numbers come first, then strings; every pass declares the same names in the same order.

Data

DOS patches (DATA at line 170)

Address, new byte, original byte, six times. Together they let DOS read the Applesoft program file as text, byte for byte:

Call table (DATA at line 2000)

136 entries, the routines compiled code calls, by index. A positive entry is an offset from the library address (2912 is $0803+2912 = $1363, the library's END); a negative entry is minus a ROM address (-57612 is $E10C, AYINT). PASS0 turns each into a stored address and puts it in SC(150) to SC(285). For a routine with a real and an integer form the table holds one of them; the other is 3 bytes away.

Operator table (DATA at line 2150)

17 operators, three numbers each: the token, its priority on the parse stack, and its priority when it arrives. ( 1/14, ) 13/1, NOT 13/14, + - 9/8, * / 11/10, ^ 13/12, AND 5/4, OR 3/2, the relations 7/6, negation 13/14. PASS1 reduces while the operator on the stack has a higher priority than the one arriving.

Page-3 machine code (DATA at line 2200)

89 bytes POKEd into $0377-$03CF (line 2240):

Symbol table chain heads (set at line 2310)

SC(0) heads the list of COMMON declarations; 1-4 are the FN chains, 5-8 the temporaries (5 integer, 6 string, 7 real, 8 inside DEF FN), 9-40 the variable chains, 41-72 the constant chains and 73-104 the line number chains. PASS0 zeroes them all. The call and operator tables lie above them, in the space where PASS1 later allocates entries: PASS1 copies the tables out before it allocates anything.