"The Structure of a Microsoft BASIC Program"

By Mike Steiner, MacTutor Contributing Editor

Unraveling the Mysteries

During a fit of boredom and out of curiosity, I started peeking with fEdit at Microsft BASIC to see how it was formatted when saved in compressed mode.

I found that if a character with the high bit set is not preceded by a quote, REM (or a single quote) or DATA, then the BASIC interpreter considers it a keyword, part of the coding of a number, or a syntax error. I received some help from Michael M. Boy of Elgin AZ, who wrote a utility that locates itself in memory (Listing 1). (Programs as presented here are for a 512K Macintosh. Change values as needed for a 128K Mac. Some experimentation may be necessary to find the correct values. When you have found the starting point, you can make the proper adjustments in listings 2 and 3.)

With this utility, we determined that a BASIC program usually starts at location 77002 (decimal) on a 512K Macintosh; however, on some occassions programs loaded four to six bytes lower in memory. Apparently, this situation happens when another application is run before BASIC is loaded. If the computer is reset, programs load at 77002. Mike then wrote a routine, which I modified (See listing 2), that prints a hex and ASCII dump of itself. The routine peeks memory from the start of the program until it finds the end of program marker. The memory dump is identical to that stored on disk with the exception that one byte is prefixed to the disk file to show the nature of the program (compressed or protected) and whether it was written in the Binary or Decimal version of BASIC. This, however, is part of another column. Following is a discussion of the format of Microsoft BASIC programs.

Format of Basic Program Lines

If a program line has n bytes, the line format is as follows:

Bytes 1 and 2: If the line is not numbered, the first byte of the line has the high bit cleared. If the line is numbered, then this bit is set. The first two bytes (high order byte first) show the length of the line; however only the second digit of the first byte is used. The maximum number of bytes in the line is normally 255. However, if there are colons or REM statements automatically inserted by BASIC (see below for a discussion of tokens automatically inserted by BASIC), the maximum number may exceed 255; the longest line I have seen had 259 bytes. The line length includes all bytes in the line, including those used internally by BASIC and not displayed in the program listing, such as the end of line marker.

The third byte is always $00.

Bytes 4 and 5: If the line is numbered, these bytes show the line number, high byte first; the highest line number is 65529. If there is no line number the body of the line starts with byte 4.

Bytes 6 (4 if no line number) through n-1: This is the information you typed in the line.

Byte n: Always $00 to show end of line. This value ($00) may appear within a line, but if it is not at position n, which is coded by the first two bytes, the program recognizes that it is not the end of line marker.

A blank line is represented by “00 04 00 00.” This includes the end of line marker. A blank numbered line is shown by “80 06 00 HB LB 00” where HB and LB are the high and low bytes of the line number.

The end of program marker is “00 00 00 00 00” including the end of line marker for the last line of the program. These five zeroes clearly describes the end of the program when the first byte of the sequence is byte n. This sequence may also appear in the body of a line as part of the coding of a declared double precision number.

Data Format Within a Line

All text within quotes or following a REM or DATA statement are represented in positive ASCII (i.e. high bit off). However, those characters that are typed in conjunction with the Option key (e.g. “Π” “÷” etc.) use negative ASCII (i.e. high bit set). Numbers are coded in positive and negative ASCII. The formatting of numbers is quite complex and is beyond the scope of this column.

Reserved words are represented by negative ASCII. There are only 128 negative ASCII bytes possible, and there are over 200 reserved words; therefore some reserved words are represented by pairs of bytes (both in negative ASCII). (See tables 1 and 2.) Any byte with high bit set that is not defined as a reserved word or part of the coding for a number and is not part of a PRINT statement, a REM, or a DATA statement is not displayed in the listing and will cause an error message when program execution reaches it.

There are a few special cases: REM, ELSE, GOTO, and GOSUB.

Special Cases

REM: Microsoft BASIC lets you use the apostrophe character as an abbreviation for REM. If you do, it inserts a $3A (colon) and an $AF (REM) before the apostrophe ($E8) token, so what is actually represented is “:REM'” When BASIC sees these three bytes, it suppresses listing the “:REM” ($3A $AF) in the list window. So, you use one extra bytes of memory whenever you use an apostrophe instead of REM at the beginning of a line (If you use REM, you need to put a space after it; with the apostrophe you do not.) Using it within the line does not use any extra bytes because if you type REM there, you have to precede it with a colon.

ELSE: Similarly, if you type ELSE in an IF - THEN statement, BASIC precedes it with a non-printing colon if you do not type one. You do not use any extra bytes in this case because your only other option is to type the colon yourself. You decide whether the colon is visible in the program listing by typing it, or not visible by letting BASIC insert it.

GOTO ($97) and GOSUB ($96) are followed by “20 1B 00 00 00” and the label name, if going to a labeled line. If going to a numbered line, the token is followed by “20 0E 00” and the line number, which is represented by two bytes, high byte first.

Managing Memory

From the above information, we can see that if available memory is a constraint, you are better off using line numbers rather than line labels in your programs. Line numbers use only two bytes in the line whereas a label uses one byte for each character in the label plus one more for the mandatory colon. Further, each reference to a labeled line elsewhere in the program uses five bytes plus the length of the label, whereas a reference to a numbered line always uses exactly five bytes. Of course, if the line is not referenced anywhere in the program, neither a label nor a line number is needed.

Description of the Goodies

Listing 1 is the locator program that finds itself in memory by searching for the REM token in the first line.

Listing 2 is the poke program that will poke the token of your choice into memory to replace a REM statement, thereby self-modifying the program. This is great for getting mathematical input and then executing it to return the value of an inputted function. This same technique was used several years ago on the Apple II by several companies to produce plot packages that could take an inputted function string and plot the results. With this utility, you can accomplish this same technique on the Macintosh.

Listing 3 is a program fragment that will do a memory dump of your program in hex and ascii.

Tables 1 is a listing of the reserved words in Microsoft Basic, sorted by ASCII code. Use this table with the poke utility to convert remark statments into new BASIC code dynamically.

Basic Listing #1: Locator Program

REM }|{here
x$ = "}|{here" : REM x$ must be the same as the REM on the above line
y$ = LEFT$(x$,1)
x = 42000! : REM start searching here, should be suitable for 128K Mac 
at this location
FOR i = x TO 512*1024
z$ = CHR$(PEEK(i))
IF z$ <> y$ THEN elp1
a$ = ""
FOR j = 1 TO LEN(x$)-1
a$ = a$ + CHR$(PEEK(i+j))
NEXT j
IF a$ = RIGHT$(x$,LEN(x$)-1) THEN PRINT "we got it at "; i : END
elp1:
IF i = x THEN PRINT "now at ";x : x = i + 1000
NEXT i

REM This program does not give the start of the program.  It gives the 
location where the first character in the REM statement begins.  Start 
of program is lower in memory.

Basic Listing #2: Poke Token in Memory

SUB printit STATIC
SHARED b
PRINT | (b) :REM the vertical bar is a place holder for the value to 
be poked and is replaced with the token for the function to executed 
by POKE 77031.  Run the program and list it again.  The vertical bar 
will be replaced by the function you selected.  DO NOT INSERT ANY TEXT 
BEFORE THE VERTICAL BAR or the program will not work.  The bar, however, 
may be replaced by any character.
END SUB

OPTION BASE 1
DIM funct (5),funct$(5)

DATA 130, 160, 181,183, 186, ATN, COS, SIN, SQR, TAN
            
FOR i = 1 TO 5: READ funct (i): NEXT
FOR i = 1 TO 5: READ funct$(i):NEXT
 
CLS
PRINT"Enter Function you want evaluated"
PRINT
PRINT"    1) ATN        2) COS        3) SIN
PRINT"    4) SQR        5) TAN

getfunction: INPUT "Your choice > ",a:  IF a<1 OR a>5 THEN getfunction
INPUT "Enter value to be processed > ",b
POKE 77031!,funct (a):REM Poke the token into memory

PRINT: PRINT
PRINT "The "; funct$(a); " of "; b; "is ";
CALL printit

Basic Listing #3:

SUB prtmem STATIC :REM Merge this routine to your program.  Then CALL 
prtmem from the command window.

CLS
CALL TEXTFONT (4)
CALL TEXTSIZE (9)
i= 77001! :REM start of program minus 1
WHILE  (PEEK(i) + PEEK(i-1) + PEEK(i-2) + PEEK(i-3) + PEEK (i-4)) <>0 
: REM Look for end of program marker  NOTE this may fail if numbers are 
declared as double precision.
k = k + 1:i = i + 1
IF k= 1 THEN PRINT USING "######"; i;:"-";
PRINT RIGHT$("0" + HEX$(PEEK(i)),2);" ";
IF PEEK(i) > 31 THEN b$ = CHR$ (PEEK(i)): ELSE b$ = "."
a$ = a$ +b$ 
IF k = 8 THEN PRINT TAB (35);a$ : k = 0 : a$ = "": REM Print 8 bytes 
then Print ASCII representation
WEND
IF k<> 0 THEN PRINT TAB (35);a$:REM Print ASCII for last line
END SUB

How This Publication was Created

Since this is our first laser printed Journal, we will join with the other Mac magazines in describing our labor pains. The text of the articles were created by the various members of the editorial staff, contributing editors and board members, using MacWrite disk version. The articles were transmitted to the editorial office over MacTerminal, in Mac-to-Mac mode, or sent US Post on floppy disk. Drawings were done in MacPaint or MacDraw, ver. 1.7. The final composition was done on Pagemaker by Aldus. The pagemaker articles were then Laser printed and sent to a web press printer. Viola! A completely electronic magazine, no paste-up!

BASIC TOKEN LIST

BY ASCII NUMBER: MICROSOFT BASIC 2.0

TOKEN DECIMAL HEX

ABS 128 80

ASC 129 81

ATN 130 82

CALL 131 83

CDBL 132 84

CHR$ 133 85

CINT 134 86

CLOSE 135 87

COMMON 136 88

COS 137 89

CVD 138 8A

CVI 139 8B

CVS 140 8C

DATA 141 8D

ELSE 142 8E

EOF 143 8F

EXP 144 90

FIELD 145 91

FIX 146 92

FN 147 93

FOR 148 94

GET 149 95

GOSUB 150 96

GOTO 151 97

IF 152 98

INKEY$ 153 99

INPUT 154 9A

INT 155 9B

LEFT$ 156 9C

LEN 157 9D

LET 158 9E

LINE 159 9F

LOC 161 A1

LOF 162 A2

LOG 163 A3

LSET 164 A4

MID$ 165 A5

MKD$ 166 A6

MKI$ 167 A7

MKS$ 168 A8

NEXT 169 A9

ON 170 AA

OPEN 171 AB

PRINT 172 AC

PUT 173 AD

READ 174 AE

REM 175 AF

RETURN 176 B0

RIGHT$ 177 B1

RND 178 B2

RSET 179 B3

SGN 180 B4

SIN 181 B5

SPACE$ 182 B6

SQR 183 B7

STR$ 184 B8

STRING$ 185 B9

TAN 186 BA

VAL 188 BC

WEND 189 BD

WHILE 190 BE

WRITE 191 BF

STATIC 227 E3

USING 228 E4

TO 229 E5

THEN 230 E6

NOT 231 E7

' (SINGLE QUOTE) 232 E8

> 233 E9

= 234 EA

< 235 EB

+ (PLUS) 236 EC

- (MINUS) 237 ED

* 238 EE

/ 239 EF

^ (CARET) 240 F0

AND 241 F1

OR 242 F2

XOR 243 F3

EQV 244 F4

IMP 245 F5

MOD 246 F6

/ 247 F7

AUTO 248 128 F8 80

CHAIN 248 129 F8 81

CLEAR 248 130 F8 82

CLS 248 131 F8 83

CONT 248 132 F8 84

CSNG 248 133 F8 85

DATE$ 248 134 F8 86

DEFINT 248 135 F8 87

DEFSNG 248 136 F8 88

DEFDBL 248 137 F8 89

DEFSTR 248 138 F8 8A

DEF 248 139 F8 8B

DELETE 248 140 F8 8C

DIM 248 141 F8 8D

EDIT 248 142 F8 8E

END 248 143 F8 8F

ERASE 248 144 F8 90

ERL 248 145 F8 91

ERROR 248 146 F8 92

ERR 248 147 F8 93

FILES 248 148 F8 94

FRE 248 149 F8 95

HEX$ 248 150 F8 96

INSTR 248 151 F8 97

KILL 248 152 F8 98

LIST 248 153 F8 99

LLIST 248 154 F8 9A

LOAD 248 155 F8 9B

LPOS 248 156 F8 9C

LPRINT 248 157 F8 9D

MERGE 248 158 F8 9E

NAME 248 159 F8 9F

NEW 248 160 F8 A0

OCT$ 248 161 F8 A1

OPTION 248 162 F8 A2

PEEK 248 163 F8 A3

POKE 248 164 F8 A4

POS 248 165 F8 A5

RANDOMIZE 248 166 F8 A6

RENUM 248 167 F8 A7

RESTORE 248 168 F8 A8

RESUME 248 169 F8 A9

RUN 248 170 F8 AA

SAVE 248 171 F8 AB

STOP 248 173 F8 AD

SWAP 248 174 F8 AE

SYSTEM 248 175 F8 AF

TIME 248 176 F8 B0

TRON 248 177 F8 B1

TROFF 248 178 F8 B2

VARPTR 248 179 F8 B3

WIDTH 248 180 F8 B4

BEEP 248 181 F8 B5

CIRCLE 248 182 F8 B6

LCOPY 248 183 F8 B7

MOUSE 248 184 F8 B8

POINT 248 185 F8 B9

PRESET 248 186 F8 BA

PSET 248 187 F8 BB

RESET 248 188 F8 BC

TIMER 248 189 F8 BD

SUB 248 190 F8 BE

EXIT 248 191 F8 BF

SOUND 248 192 F8 C0

BUTTON 248 193 F8 C1

MENU 248 194 F8 C2

WINDOW 248 195 F8 C3

DIALOG 248 196 F8 C4

LOCATE 248 197 F8 C5

CSRLIN 248 198 F8 C6

LBOUND 248 199 F8 C7

UBOUND 248 200 F8 C8

SHARE 248 201 F8 C9

UCASE$ 248 202 F8 CA

SCROLL 248 203 F8 CB

LIBRARY 248 204 F8 CC

CVSBCD 248 205 F8 CD

CVDBCD 248 206 F8 CE

MKSBCD$ 248 207 F8 CF

MKDBCD$ 248 208 F8 D0

OFF 249 244 F9 F4

BREAK 249 245 F9 F5

WAIT 249 246 F9 F6

USR 249 247 F9 F7

TAB 249 248 F9 F8

STEP 249 249 F9 F9

SPC 249 250 F9 FA

OUTPUT 249 251 F9 FB

BASE 249 252 F9 FC

AS 249 253 F9 FD

APPEND 249 254 F9 FE

ALL 249 255 F9 FF

PICTURE 250 128 FA 80

WAVE 250 129 FA 81

LINETO 251 94 FB 5E

FILLPOLLY 251 210 FB D2

INVERTPOLY 251 211 FB D3

ERASEPOLY 251 212 FB D4

PAINTPOLY 251 213 FB D5

FRAMEPOLY 251 214 FB D6

PTAB 251 215 FB D7

FILLARC 251 216 FB D8

INVERTARC 251 217 FB D9

ERASEARC 251 218 FB DA

PAINTARC 251 219 FB DB

FRAMEARC 251 220 FB DC

FILLROUNDRECT 251 221 FB DD

INVERTROUNDRECT 251 222 FB DE

ERASEROUNDRECT 251 223 FB DF

PAINTROUNDRECT 251 224 FB E0

FRAMEROUNDRECT 251 225 FB E1

FILLOVAL 251 226 FB E2

INVERTOVAL 251 227 FB E3

ERASEOVAL 251 228 FB E4

PAINTOVAL 251 229 FB E5

FRAMEOVAL 251 230 FB E6

FILLRECT 251 231 FB E7

INVERTRECT 251 232 FB E8

ERASERECT 251 233 FB E9

PAINTRECT 251 234 FB EA

FRAMERECT 251 235 FB EB

TEXTSIZE 251 236 FB EC

TEXTMODE 251 237 FB ED

TEXTFACE 251 238 FB EE

TEXTFONT 251 239 FB EF

MOVE 251 241 FB F1

MOVETO 251 242 FB F2

PENNORMAL 251 243 FB F3

PENPAT 251 244 FB F4

PENMODE 251 245 FB F5

PENSIZE 251 246 FB F6

GETPEN 251 247 FB F7

SHOWPEN 251 248 FB F8

HIDEPEN 251 249 FB F9

OBSURECURSOR 251 250 FB FA

SHOWCURSOR 251 251 FB FB

HIDECURSOR 251 252 FB FC

SETCURSOR 251 253 FB FD

INITCURSOR 251 254 FB FE

BACKPAT 251 255 FB FF