Difference between revisions of "Category:Model 100 Classics"

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How does Basic do what you tell it to do? Clues to the language's subservience lie in ROM.
 
  
<big>By JAKE COMMANDER</big>
 
 
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 unravelled 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 <code>TAB(22);l/3</code>, so would an <code>LPRINT</code>. But a LET would have none of that. <code>LET X = TAB(22); 1/3</code> 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 <code>OFF</code>, which can be <code>SOUND OFF</code> or <code>MOTOR OFF</code> 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 <code>MID$(LH$)=RH$</code>, and <code>LH$=MID$(RH$)</code>. 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: signed integer, single precision, double precision, and 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.
 
 
JAKES 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 lO0 September 1983
 

Revision as of 22:52, 22 January 2009

Pages in category "Model 100 Classics"

The following 4 pages are in this category, out of 4 total.