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⌂ ∞ ▽ Elite on the 6502

Version analysis of BULB

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

       Name: BULB
       Type: Subroutine
   Category: Dashboard
    Summary: Draw an indicator bulb on the dashboard



 Arguments:

Code variation 1 of 3A variation in the comments only

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked
Electron
   A                    The y-coordinate of the bulb as a low-byte screen
                        address offset within screen page &7D (as both bulbs
                        are on this character row in the dashboard)

   (Y X)                The address of the character definition of the bulb to
                        be drawn (i.e. ECBT for the E.C.M. bulb, or SPBT for the
                        space station bulb)

   A                    The low byte of the screen address of the bulb to show

   X                    The low byte of the address of the character definition
                        of the bulb to be drawn, i.e. #LO(ECBT) for the E.C.M.
                        bulb, or #LO(SPBT) for the space station bulb

   Y                    The high byte of the screen address of the bulb to show

Code variation 2 of 3A variation in the comments only

Cassette, Flight, Docked, Electron
 Other entry points:

   BULB-2               Set the Y screen address


.BULB

 STA SC                 \ Store the low byte of the screen address in SC

Code variation 3 of 3Related to the screen mode

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked
Electron
 STX P+1                \ Set P(2 1) = (Y X)
 STY P+2

 LDA #&7D               \ Set A to the high byte of the screen address, which is
                        \ &7D as the bulbs are both in the character row from
                        \ &7D00 to &7DFF

 STX P+1                \ Set P(2 1) to the address of the character definition
 LDX #HI(ECBT)          \ of the bulb to be drawn (this assumes that ECBT and
 STX P+2                \ SPBT are in the same page and have the same high byte)

 TYA                    \ Set A to Y, the high byte of the screen address we
                        \ want to write to, so now (A SC) points to the specific
                        \ bulb's screen address

 JMP RREN               \ Call RREN to print the character definition pointed to
                        \ by P(2 1) at the screen address pointed to by (A SC),
                        \ returning from the subroutine using a tail call