Dashboard: SPBLB

       Name: SPBLB                                                   [Show more]
       Type: Subroutine
   Category: Dashboard
    Summary: Light up the space station indicator ("S") 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:
             * KS4 calls SPBLB
             * NWSPS calls SPBLB
             * RES2 calls SPBLB
             * ECBLB calls via away
             * HANGER calls via away
             * MSBAR calls via away


 Other entry points:

   away                 Switch main memory back into &3000-&7FFF and return from
                        the subroutine


.SPBLB

 LDA #%00001111         \ Set bits 1 and 2 of the Access Control Register at
 STA VIA+&34            \ SHEILA &34 to switch screen memory into &3000-&7FFF

 LDA #16*8              \ The space station bulb is in character block number 48
 STA SC                 \ (counting from the left edge of the screen), with the
                        \ first half of the row in one page, and the second half
                        \ in another. We want to set the screen address to point
                        \ to the second part of the row, as the bulb is in that
                        \ half, so that's character block number 16 within that
                        \ second half (as the first half takes up 32 character
                        \ blocks, so given that each character block takes up 8
                        \ bytes, this sets the low byte of the screen address
                        \ of the character block we want to draw to

 LDA #&7B               \ Set the high byte of SC(1 0) to &7B, as the bulbs are
 STA SC+1               \ both in the character row from &7A00 to &7BFF, and the
                        \ space station bulb is in the right half, which is from
                        \ &7B00 to &7BFF

 LDY #15                \ Now to poke the bulb bitmap into screen memory, and
                        \ there are two character blocks' worth, each with eight
                        \ lines of one byte, so set a counter in Y for 16 bytes

.BULL2

 LDA SPBT,Y             \ Fetch the Y-th byte of the bulb bitmap

 EOR (SC),Y             \ EOR the byte with the current contents of screen
                        \ memory, so drawing the bulb when it is already
                        \ on-screen will erase it

 STA (SC),Y             \ Store the Y-th byte of the bulb bitmap in screen
                        \ memory

 DEY                    \ Decrement the loop counter

 BPL BULL2              \ Loop back to poke the next byte until we have done
                        \ all 16 bytes across two character blocks

.away

 LDA #%00001001         \ Clear bits 1 and 2 of the Access Control Register at
 STA VIA+&34            \ SHEILA &34 to switch main memory back into &3000-&7FFF

 RTS                    \ Return from the subroutine