The BBC and Master Computer Public Domain Library

AE74 RIGHT$(

Submitted by Steve Fewell

Description:

On entry carry flag is set if the RIGHT$ function was called (&AE74); otherwise, if carry is clear then we were entered
from the LEFT$ function (&AE73).
Store the entry status (carry flag) to the 6502 processor stack.
Call routine &9D3B to evaluate the expression at BASIC text pointer B and issue a 'Type mismatch' error if a String
value was not returned.
If the next non-space character (stored in X) is not a comma then issue 'Missing ,' error; otherwise, increment BASIC text
pointer B offset to skip past the comma value.

Push the SWA value to the BASIC Stack, get the Integer result of the expression at BASIC Text pointer B ('Type mismatch'
error if value is not numeric) and check for a closing ')' character ('Missing )' error if closing parenthesis not found.

Retrieve the String value (SWA) from the BASIC Stack.
Retrieve the carry flag routine entry setting from the 6502 processor stack.

If carry is not set (LEFT$) then:

* If the Integer value (&2A) is greater than or equal to the length of the String value in the SWA (&36) then exit

   with A = 0 (as the result is a String value).

* Otherwise, Set the SWA String length (&36) to the Integer value (to reduce the SWA length to the width specified

   and exit with A = 0 (as the result is a String value).

If carry is set (RIGHT$) then:

* Subtract the Integer value (&2A) from the SWA length (&36) [Note that carry flag is automatically set before

   subtraction].

* If the subtraction underflowed (the Integer value is greater than of equal to the SWA length) then exit with A = 0

   (as the result is a String value).

* Set X to the result (the first character of the result value).

* Set the SWA length (&36) to the Integer value specified (&2A).

* If the new SWA length is 0 then exit.

* Set Y = 0 and keep copying the character at SWA location X to SWA location Y and then incrementing Y and X, and

   decrementing &2A until &2A is 0 (the required number of characters has been copied).

* Exit with A = 0 (as the result is a String value located in the SWA).

Disassembly for the RIGHT$( routine

Disassembly for the Push SWA to stack, get Integer result of expression & check for closing ')' routine