The BBC and Master Computer Public Domain Library

BDE5 Check program can be read correctly

Submitted by Steve Fewell

Description:

Set TOP (&12-&13) to PAGE (&18) [The LSB byte of TOP is zero as PAGE does not store an LSB byte].

[&BDED] If the first byte of the program line is not #&0D then show the 'Bad program' message [described below].

If the second byte of the program line is negative then we have reached the end of the program (#&FF value), so add 2
to the TOP value to account for the 2-byte null line value and exit with Y = 1. The program was checked correctly!

If the fourth byte of the program line (line length) is #&00 then the program is corrupt, as this is an invalid line
length, so show the 'Bad program' message [described below - &BE11].

[&BDFE] Otherwise, the program line is ok, so Add the line length to TOP and set Y back to 1 (offset for next line);
then, jump back to &BDED to check the next program line.

[&BE11] Show 'Bad program' message
This routine is called if the program check located an error or corruption in the program, meaning that the program
is not correct. The text 'Bad program' will be shown on the screen - this is only a screen message and not a forced
error that stops the current operation and jumps to a specified error routine - specified by the BBC's Break [BRK] vector.

This routine call routine &BECF.
Routine &BECF does the following:
* Retrieves the address of the position to return to (after the subroutine finishes) from the stack.
* Store this address in (&37, &38). Now, (&37,. &38) point to the first character after the &BECF call.
* Keep on incrementing (&37, &38), reading the character from this location and, if the character is positive
(less than #&80), then:
      -> Show the character on the screen [if the characetr is #&0D then a new line will be shown instead]
      -> Jump back to increment (&37, &38) and process the next character.
* If the character is negative (greater than or equal to #&80) then there are no more characters to display, so
jump to the current location pointed to by (&37, &38) -> this will be the location directly after the text message.
[The first character after the text message is usually #&EA (The NOP Mnenomic), which represents the end of the text,
as its value is negative (>=&80].

In the case of the 'check program can be read correctly' routine, the program code after the Text message is:
NOP
JMP &8F86

The NOP command does nothing (but as it has a value >= #&80, it represents the end of the message).
The JMP command jumps to &8F86 which returns to the BASIC command line to read and execute the user's next command
line instruction.

Note: This routine does not need to end with an RTS, as the stack has already been tidied up by the removal of the return
address bytes.

Disassembly for the Check program can be read correctly routine

Output text warning message and jump to the address specified