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Elite on the BBC Micro

Dashboard: SPBLB [Master version]

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 entry point BULB2 * HANGER calls entry point BULB2 * MSBAR calls entry point BULB2

Other entry points: BULB2 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 .BULL 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 BULL \ Loop back to poke the next byte until we have done \ all 16 bytes across two character blocks .BULB2 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