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⌂ ∞ ▽ Elite on the BBC Micro and NES

Maths (Arithmetic): LCASH

[Elite-A, Docked]

       Name: LCASH                                                   [Show more]
       Type: Subroutine
   Category: Maths (Arithmetic)
    Summary: Subtract an amount of cash from the cash pot

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * cour_buy calls LCASH
             * eq calls LCASH
             * stay_here calls LCASH
             * TT219 calls LCASH




 Subtract (Y X) cash from the cash pot in CASH, but only if there is enough
 cash in the pot. As CASH is a four-byte number, this calculates:

   CASH(0 1 2 3) = CASH(0 1 2 3) - (0 0 Y X)



 Returns:

   C flag               If set, there was enough cash to do the subtraction

                        If clear, there was not enough cash to do the
                        subtraction



.LCASH

 STX T1                 \ Subtract the least significant bytes:
 LDA CASH+3             \
 SEC                    \   CASH+3 = CASH+3 - X
 SBC T1
 STA CASH+3

 STY T1                 \ Then the second most significant bytes:
 LDA CASH+2             \
 SBC T1                 \   CASH+2 = CASH+2 - Y
 STA CASH+2

 LDA CASH+1             \ Then the third most significant bytes (which are 0):
 SBC #0                 \
 STA CASH+1             \   CASH+1 = CASH+1 - 0

 LDA CASH               \ And finally the most significant bytes (which are 0):
 SBC #0                 \
 STA CASH               \   CASH = CASH - 0

 BCS TT113              \ If the C flag is set then the subtraction didn't
                        \ underflow, so the value in CASH is correct and we can
                        \ jump to TT113 to return from the subroutine with the
                        \ C flag set to indicate success (as TT113 contains an
                        \ RTS)

                        \ Otherwise we didn't have enough cash in CASH to
                        \ subtract (Y X) from it, so fall through into
                        \ MCASH to reverse the sum and restore the original
                        \ value in CASH, and returning with the C flag clear