This text file explains what the source code does, line by line.

REM $option k10
REM $option y+,v+,u+,[,]
REM $option fNUMGAME.TOS

These three lines set the options flags. The k option tells HBASIC how 
much memory to give to the program. The y+ is to get rid of the main 
window, v+ to check variables, u+ to underlines in variables, the "[" 
to select array warnings and "]" to allow undefined sub-programs. The 
'f' option sets the name of the output file when compiling.

DEF FNgen
STATIC y

DO
y=CINT(RND*1000)
LOOP UNTIL y<1000 AND y>0
FNgen=y

END DEF

Our function called gen. This routines makes sure that a number 
generated with RND (which is always below 1, so it's multiplied by 
1000 to get a decent number, and rounded off with the CINT function) 
is put into y. This routines makes sure the number is below 1000 but 
above 0.

SUB game
STATIC num,low,high,turn,a,a$

RANDOMIZE

The RND function would normally give the same number all the time, but 
this RANDOMIZE function gives a new lot of numbers for the RND to use.

num=FNgen

turn=0
low=1
high=999

Get our generated number and put it in num, also, make sure the 
options here (turn is the number of turns, low is the lowest number 
range and high is the highest number range).

DO
INCR turn
PRINT "Turn:";turn
PRINT "The number is between";low;"and";high
PRINT

Display our info on turn number, low and high ranges.

INPUT "What do you think the number is? ",a

IF a>999 THEN STOP
IF a<num THEN PRINT "The number is greater than";a : IF a>low THEN low=a
IF a>num THEN PRINT "The number is below";a : IF a<high THEN high=a
IF a=num THEN
	PRINT
	PRINT "Congratulations! The number was";num;"and you guessed correctly in";turn;"turns!" : a=1000
	PRINT
END IF

The the user's number and put it into a. Then we have a check to see 
if the number is above/below the generated one. If the generated 
number is greater than a, the program will check to see if the user's 
number is above the lowest range, and if it is it will change the 
lowest to that number. It also checks the number for the highest too. 
If you get the right number, it tells you congratulations and tells 
you the number and how many turns it took.

PRINT 
LOOP UNTIL a>999

PRINT "Would you like another game?"

DO
a$=UCASE$(INKEY$)
LOOP UNTIL a$="Y" OR a$="N"

IF a$="Y" THEN CALL game

END SUB

Once finished a game, the computer displays "Would you like another 
game?". The above code involving the INKEY$ gets a key from the 
keyboard. The UCASE$ turns this code into upper case, if I pressed 'y' 
then the computer would turn this to 'Y'. If the answer is yes, it 
will call the routine again.

PRINT "Paul's number guessing game. "+CHR$(189)+" Copyright to Paul Jones and Atari Computing"
PRINT "1997. Example program using SUB/FUNCTIONS and maths functions as in"
PRINT "tutorial #3."

CALL game
STOP -1

Displays information at the start of the program, and then calls the 
main game sub routine. The STOP -1 tells the program to quit (once the 
main game has finished).