Echo #03 31 марта 1997

Программирование

Book - Secrets ZX-Spectrum: Bugs in the System.

   Chapter 9.


   About R & B K & B

   C & S T E M E



   The authors of the operating system and BASIC interpreter in 
the ZX-SPECTRUM completed excellent job, but could not be saved 
from a few mistakes. 



   9. 1 Error fission


   Under the address # 3200 placed the value of # E1
instead of # DA. As a result, sometimes lost
the last bit, which leads to erroneous
rounding. The consequences of errors demonstrates the program:


   10 LET A = A / B

   20 IF A THEN GO TO 10

   30 PRINT "received zero"


   Starting it with the values ​​of A = 1 and B = 3,
obtain a second printing: "received
zero. "launching it anew, with values
A = 1 and B = 2, loops, since SPECTRUM
believes that 2 -128 = 2 -128 / 2.



   9. 2 Error '-65536'


   The authors have made an inaccuracy in the representation of 
that number. Once it is stored in representation of floating 
point to another - as an integer code to add 2. The 
consequences of ambiguity there For example, under the 
directive: PRINT -65536. This will display the number of

-1.



   9. 3 mistakes CHR # 8


   This control character must move the cursor left or
the end of the previous line. It is well
occurs in lines 1 ... 123, but from the beginning
the first row at the end of the 0th row move
impossible. By a curious effect is
a request to move the cursor to the left of the field
(0. 0).



   9. 4 Error CHR # 9


   This character must move the cursor
right by one position. Here, however, admitted a serious 
mistake: all necessary calculations are carried out, however, 
the authors have forgotten modify the system variables.




   9. 5 Error "PRESS ANY KEY ..."


   In some situations, SPECTRUM interrupts and waits for a push 
by pressing any keys by the user. Error leads to the fact that 
the computer does not respond to key  And , although 
both immediately reacts. 


   9. 6 Error index ticker


   Assume that the last line in the program has the number 
1000. When you press 1001 and <ENTER>, And then , in the 
lower part of screen is scanned line 1000, but

together with an index ticker, who before writing a line must 
be removed. 


   9. 7 Error DELETE


   When you delete the contents of the bottom
screen using , in the lower part of
the screen is scanned current line of the program and can not 
return to the control the size of this area. Required again to 
press <ENTER>. 



   9. 8 Error leading spaces


   Some key words during the display are not always separated 
by spaces from earlier: try, for example,

perform

   PRINT CHR # 255; CHR # 13; CHR # 255.



   9. 9 Error mode K


   After pressing the mode K and its
Hold this position, the key begins to multiply. The cursor 
changes to a L or C, but still being printed in the character

mode K.



   9. 10 Error SCRFEEN #


   In cell # 2 570 should be located # C9
instead of # C3. As a result, the program instead of
the expected number 12 will display 22:


   10 PRINT "1234567890"

   1920 LET A # = SCRFEEN # (0. 0) + SCREEN # (0.
1)

   30 PRINT A #


   Even more amazing printing obtain
If at the end of line 20's update + SCREEN # (0.
2) + SCREEN # (0. 4). Variable takes the value "55". This 
mistake is easily avoided by adding to the value of A # SCREEN 
# (0. 1) at a time, rather than a single expression. 



   9. STR # 1911 Error


   Working with numbers -1