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⌂ ∞ ▽ Elite on the BBC Micro and NES

Dashboard: ECBLB

[BBC Micro cassette version]

       Name: ECBLB                                                   [Show more]
       Type: Subroutine
   Category: Dashboard
    Summary: Light up the E.C.M. indicator bulb ("E") on the dashboard

    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: This subroutine is called as follows:
             * ECMOF calls ECBLB




.ECBLB

 LDA #7*8               \ The E.C.M. bulb is in character block number 7
                        \ with each character taking 8 bytes, so this sets the
                        \ low byte of the screen address of the character block
                        \ we want to draw to

 LDX #LO(ECBT)          \ Set (Y X) to point to the character definition in
 LDY #HI(ECBT)          \ ECBT. The LDY has no effect, as we overwrite Y with
                        \ the jump to BULB-2, which writes the high byte of SPBT
                        \ into Y. This works as long as ECBT and SPBT are in
                        \ the same page of memory, so perhaps the BNE below got
                        \ changed from BULB to BULB-2 so they could remove the
                        \ LDY, but for some reason it didn't get culled? Who
                        \ knows...

 BNE BULB-2             \ Jump down to BULB-2 (this BNE is effectively a JMP as
                        \ A will never be zero)