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⌂ ∞ ▽ Aviator on the BBC Micro

Graphics: ClearRows

       Name: ClearRows                                               [Show more]
       Type: Subroutine
   Category: Graphics
    Summary: Clear the specified number of rows on-screen

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawCanopy calls ClearRows




 This routine zeroes a block of Y bytes on R screen rows, starting from screen
 address (Q P) on the first row.

 A value of Y = 0 will zero 256 bytes.

 In other words, (Q P) represents the top-left pixel to blank, Y represents
 the width of the area to blank (with a value of 8 per character block), and R
 contains the number of rows to blank.



 Arguments:

   Y                    The width of each character row to zero (in bytes),
                        0 indicates 256 bytes

   (Q P)                The screen address to start zeroing from

   R                    The number of character rows to zero



.ClearRows

 STY S                  \ Store the width of each character row in S

.crow1

 LDA #0                 \ We are about to zero a block of memory, so set A = 0
                        \ so we can use it as our overwrite value

 LDY S                  \ Fetch the width of each character row, which we stored
                        \ in S above

.crow2

 STA (P),Y              \ Zero the Y-th byte of the page at (Q P), which sets 4
                        \ pixels to black

 DEY                    \ Decrement the byte pointer

 BNE crow2              \ Loop back until we have zeroed (Q P) to (Q P) + Y

 LDA P                  \ Set (Q P) = (Q P) + 320
 CLC                    \
 ADC #LO(320)           \ starting with the low bytes
 STA P

 LDA Q                  \ And then the high bytes
 ADC #HI(320)
 STA Q

 DEC R                  \ Decrement the row counter in R

 BNE crow1              \ Loop back until we have zeroed R rows

 RTS                    \ Return from the subroutine

 SKIP 30                \ These bytes appear to be unused
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 EQUB &FF, &FF
 SKIP 32
 EQUB &40