ROM Addresses: Getting BASIC to do What You Want

How does BASIC do what you tell it to do? Clues to the language's subservience lie in ROM.

By JAKE COMMANDER
Portable l00 September 1983

How on earth does Basic know what to do? All those state­ments, commands, and functions, yet the interpreter untiringly plods through your code always knowing what's required next. Just how does it do it?

Well, if the answer were simple, everybody would be writing BASIC interpreters and putting Microsoft out of business. But it is possible to follow at least some of the pathways Basic uses to perform its duties.

Most addresses of the ROM routines which comprise BASIC are held in two tables. These can be unraveled to give a list of routines used to perform various tasks.

Jump Addresses

One table contains jump addresses for the commands (or verbs, as it were) which will always be the first thing the interpreter picks up from a statement. The whole repertoire of such commands is catered for the table located at 0262 hex.

BASIC gets the appropriate jump address by using the token number for the command it's about to execute. All tokens are numbers from 128 to 255; therefore subtracting 128 gives numbers from zero to 127. As each jump address in the table is two bytes long, the token (minus 128) is multiplied by two to give an offset into the table. This points straight at the address which is needed. The two-byte address is picked up and jumped to ? and we're now executing a BASIC command in pure machine code.

What happens next depends entirely on the machine code for the command itself. Various syntaxes are allowed for some commands but not for others. For instance, the print command would allow an expression such as TAB(22);l/3, so would an LPRINT. But a LET would have none of that. LET X = TAB(22); 1/3 would have you on the carpet in no time.

Also various combinations of tokens can do different things. The comparison operators, for example, can be used pretty much interchangeably. These operators, >? <> = < , etc are all OK syntactically. This versatility means a table for such a wide set of possibilities is nigh impossible.

SECOND TABLE

However, there is a second table at location 004E in the ROM. This contains many addresses used in the evaluation of Basic math functions and expressions. These are extracted and jumped to in a similar fashion to the first table.

Any BASIC word excluded from either of these tables is handled separately by the interpreter according to its particular use. However, out of a possible 128 tokens, these two tables give us a mechanism by which we can follow the machine-code execution of many of them. It is the combination of these routines and the syntax checking required to logically execute them that makes up an interpreter.

The following list has been compiled from the two tables I've described and a disassembly of other parts of the ROM. It shows the entry points for all important BASIC statements and functions. Certain functions can have more than one possible syntactic use and the list does not cover all such uses. (An example is the statement OFF, which can be SOUND OFF or MOTOR OFF etc.) The list is in four columns. The first is the address in ROM where the BASIC word occurs in the vocabu­lary table. The second entry is the word itself. Third is the token assigned to that word when it is encoded by the BASIC interpreter.

FOURTH COLUMN

The fourth column contains the address the interpreter jumps to to execute the token representing the statement or function desired. Once again, some statements can have more than one use such as MID$(LH$)=RH$, and LH$=MID$(RH$). In these cases, two addresses are given: one for use on the left hand side of the equals sign and one for the right hand side of the sign.

Perhaps unsurprisingly, things get a little more complicated with the mathematical functions in BASIC. It's not simply a matter of taking an address for, say, a multiply routine and then jumping to it. The BASIC interpreter has to know the numeric type of operator it has to work on. For instance, with the addition operator, BASIC has four choices:

1. Signed integer
2. Single precision
3. Double precision
4. String

None of the other binary operators allow string manipulation, so they're limited to the numeric variable types only.

The addresses of these binary operators can be confirmed (if you need confirmation) from three short tables in ROM, one each for double precision, single precision, and integer numbers respectively.

The tables contain six addresses apiece for addition, subtraction, multiplication, division, exponentiation, and comparison. Rather than clutter the token-addresses table, these addresses are contained separately at the end.

In a following article, I'll be looking at ways to use some of these addresses in your own machine-code programs. For the more adventur­ous, an experiment will probably prove irresistible. Remember, though, in a RAM-file machine such as the Model 100, a lock-up may cost you all your files. Use caution.

JAKE'S ROM ADDRESSES FOR BASIC KEYWORDS

0080 => END = 80 @ 409F

0083 => FOR = 81 @ 0726

0086 => NEXT = 82 @ 4174

008A => DATA = 83 @ 099E

008E => INPUT = 84 @ 0CA3

0093 => DIM = 85 @ 478B

0096 => READ = 86 @ OCD9

009A => LET = 87 @ 09C3

009D => GOTO = 88 @ 0936

OOA1 => RUN = 89 @ 090F

OOA4 => IF = 8A @ OB1A

OOA6 => RESTORE = 8B @ 407F

00AD => GOSU8 = 8C @ 091E

0082 => RETURN = 8D @ 0966

OOB8 => REM = 8E @ 09AO

OOBB => STOP = 8F @ 409A

OOBF => WIDTH = 9D @ 1DC3

OOC4 => ELSE = 91 @ 09AO

OOC8 => LINE = 92 @ OC45

OOCC => EDIT = 93 @ 5E51

OODO => ERROR = 94 @ OBOF

00D5 => RESUME = 95 @ OABO

OODB => OUT = 96 @ 11OC

OODE => ON = 97 @ OA2F

OOEO => DSKOS = 98 @ 5071

OOE5 => OPEN = 99 @ 4CCB

OOE9 => CLOSE = 9A © 4E20

OOEE => LOAD = 98 @ 4D70

OOF2 => MERGE = 9C @ 4D71

OOF7 => FILES = 9D @ 1F3A

OOFC => SAVE = 9E © 4DCF

0100 => LFILES = 9F @ 506F

0106 => LPRINT = AO @ OB4E

010C => DEF = A1 @ 0872

010F => POKE = A2 @ 128B

0113 => PRINT = A3 @ 0B56

0118 => CONT = A4 @ 40DA

011C => LIST = A5 @ 1140

0120 => LLIST = A6 @ 113B

0125 => CLEAR = A7 @ 40F9

012A => CLOAD = A8@ 2377

012F => CSAVE = A9 @ 2280

0134 => T1ME$ = AA @ 19AB 1904

0139 => DATES = AB @ 19BD 1924

013E => DAYS = AC © 19F1 1955

0142 => COM = AD @ 1A9E

0145 => MDM = AE @ 1A9E

0148 => KEY = AF @ 1BB8

014B => CLS = BO @ 4231

014E => BEEP = B1 @ 4229

0152 => SOUND = B2 @ 1DC5

0157 => LCOPY = B3 @ 1E5E

015C => PSET = B4 @ 1C57

0160 => PRESET = B5 @ 1C66

0166 => MOTOR = B6 @ 1DEC

016B => MAX = B7 @ 7FOB 19DB

016E => POWER = B8 @ 1419

0173 => CALL = B9 @ 1DFA

0177 => MENU = BA @ 5797

017B => IPL = BB @ 1A78

017E => NAME = BC @ 2037

0182 => KILL = BD @ 1F91

0186 => 3CREEN = BE @ 1E22

018C => NEW = BF @ 20FE

018F => TAB( = CO @ OC01  ? 0193 => TO = C1 @ 076B

0195 => USING = C2@ 4991

D19A => VARPTR = C3 @ OF7E

01AO => ERL = C4@ OF56

01 A3 => ERR = C5@ OF47

01 A6 => STING$ = C6@ 296D

01 AD => INSTR = C7 @ 2A37

0182 => DSKI$ = C8 @ 5073

01B7 => INKEY$ = C9@ 4BEA

01BD => CSRLIN = CA@ 1D90

01C3 => OFF = C8 @ various

01C6 => HIMEM = CC @ 1DB9

01CB => THEN = CD® OB2A

01CF => NOT = CE @ 1054

01D2 => STEP = CF @ O783

01 06 => + = D0 *

01D7 => - = D1 * See

01D8 => * = D2 * table

01 D9 => / = D3 * at end

010A => /\ = D4 *

01 DB => AND = D5 @ 1097

01 DE => OR = 06 @ 108C

O1EO => XOR = D7 @ 10A2

01E3 => EQV = D8 @ 10AD

01E6 => IMP = D9 @ 10B5

01E9 => MOD = DA@ 37DF

01 EC => \ = DB @ 377E

01ED => > = DC@ OE29

01EE => = = DD@ OE29

01EF => < = DE @ OE29

01FO => SGN = DF @ 3407

01F3 => INT = E0 @ 3654

OIF6 => ABS = E1 @ 33F2

01F9 => FRE = E2 @ 2B4C

01FC => INP = E3 @ 1100

01FF => LPOS = E4 @ 10C8

0203 => POS = E5 @ 10CE

0206 => SQR = E6 @ 305A

0209 => RND = E7 @ 313E

020C => LOG = E8 @ 2FCF

020F => EXP = E9 @ 30A4

0212 => COS = EA @ 2EEF

0215 => SIN = EB @ 2F09

0218 => TAN = EC @ 2F58

021B => ATN = ED @ 2F71

021E => PEEK = EE @ 1284

0222 => EOF = EF @ 1889

0225 => LOG = F0 @ 506D

0228 => LOF = F1 @ 506B

022B => CINT = F2 @ 3501

022F => CSNG = F3 @ 352A

0233 => CDBL = F4 @ 35BA

0237 => FIX = F5 @ 3645

023A => LEN = F6 @ 2943

023D => STR$ = F7 @ 273A

0241 => VAL = F8 @ 2A07

0244 => ASC = F9 @ 294F

0247 => CHR$ = FA @ 295F

0248 => SPACE$ = FB @ 298E

0251 => LEFT$ = FC @ 29AB

0256 => RIGHT$ = FD @ 29DC

025C => MID$ = FE @ 2AC2 29E6

0260 => ' = FF @ OA90

+ - * / > Cmpr D.P. 2B78 2B69 2CFF 2DC7 3D8E 34FA S.P. 37F4 37FD 3803 380E 3D7F 3498 INT 3704 36F8 3725 OFOD 3DF7 34C2 Siring 28CC 270C

Portable l00 September 1983