Graphics: DrawGunSights

       Name: DrawGunSights                                           [Show more]
       Type: Subroutine
   Category: Graphics
    Summary: Draw the canopy corners and the gun sights, if shown
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawCanopyView calls DrawGunSights


.DrawGunSights

 JSR DrawCanopyCorners  \ Draw the canopy corners

 LDX #&DA               \ Scan the keyboard to see if "I" is being pressed
 JSR ScanKeyboard

 BNE guns1              \ If "I" is not being pressed, jump to guns1

 BIT gunSights          \ If bit 6 of gunSights is set, then "I" is still being
 BVS guns3              \ held down from a previous call to this routine, so
                        \ jump to guns3 to move on from the subroutine

 LDA #%11000000         \ Set A = gunSights with bits 6 and 7 flipped, which we
 EOR gunSights          \ will set as the new value of gunSights below

 BMI guns2              \ If the result has bit 7 set, that means that bit 7 of
                        \ gunSights is currently clear, so jump to guns2 as the
                        \ sights are not already being shown, so we don't need
                        \ to remove them

                        \ If we get here then bit 7 of gunSights is set, so the
                        \ sights are already on-screen, and we aren't still
                        \ holding down "I" from a previous call, so now we
                        \ remove the sights

 PHA                    \ Store A on the stack so we can retrieve it below

 LDY #HI(row6_char1_0)  \ Set (Y X) to the screen address for row 6, character
 LDX #LO(row6_char1_0)  \ block 1

 LDA #3                 \ Set R = 3, so we clear 3 character rows for the sights
 STA R

 LDA #0                 \ Set X = 0 so we clear the sights to black

 JSR FillCanopyRows     \ Fill the 3 screen rows with black, avoiding the canopy
                        \ edges and removing the sights from the screen

 PLA                    \ Retrieve A from the stack

 JMP guns2              \ Jump down to guns2 to set gunSights to the value in A

.guns1

                        \ If we get here then "I" is not being pressed

 LDA #%10000000         \ Extract bit 7 from gunSights into A, which clears bit
 AND gunSights          \ 6 to indicate that "I" is not being pressed, while
                        \ leaving the current state of bit 7 alone

.guns2

 STA gunSights          \ Update gunSights to the new value in A

.guns3

 BPL ToggleJoystick     \ If bit 7 of the new value of gunSights is clear, then
                        \ the sights are no longer being shown, so jump down to
                        \ ToggleJoystick to move on to the next routine

                        \ If we get here, then bit 7 of gunSights is set, so we
                        \ need to draw the sights

 LDY #7                 \ First we draw the vertical bar in the middle of the
                        \ sights, which runs through block 20 on rows 6 and 7,
                        \ so we set a pixel row counter in Y to count through
                        \ the rows in each character block

.guns4

 LDA #%10001000         \ Draw a vertical bar in the first pixel of the Y-th
 ORA row6_char20_0,Y    \ pixel row in character block 20 on row 6, keeping
 STA row6_char20_0,Y    \ whatever is already on screen

 LDA #%10001000         \ Draw a vertical bar in the first pixel of the Y-th
 ORA row7_char20_0,Y    \ pixel row in character block 20 on row 7, keeping
 STA row7_char20_0,Y    \ whatever is already on screen

 DEY                    \ Decrement the pixel row counter

 BPL guns4              \ Loop back to draw the next pixel row until we have
                        \ drawn a vertical line through both character blocks

                        \ Now to draw the horizontal bar of the gun sights

 LDA #%01110111         \ First we draw the left end of the horizontal bar,
 ORA row8_char11_0      \ which is starts with the three rightmost pixels in
 STA row8_char11_0      \ character block 11 on row 8

 SEC                    \ Set the C flag for the subtraction below

 LDY #136               \ The rest of the line is made up of 17 character blocks
                        \ with four pixels in each block, so we set a counter in
                        \ Y to work as an offset from the left end of the line,
                        \ counting 8 bytes per character block (17 * 8 = 136),
                        \ working from the right end of the line to the left

.guns5

 LDA #%11111111         \ Draw a horizontal bar of four pixels starting at the
 STA row8_char11_0,Y    \ Y-th offset from the start of character block 11 on
                        \ row 8

 TYA                    \ Set Y = Y - 8, so Y now points to the four pixels to
 SBC #8                 \ the left, as we work or way along the line
 TAY

 BNE guns5              \ Loop back to draw the next four pixels of the line
                        \ until we have drawn the whole horizontal line

                        \ Fall through into ToggleJoystick to check for the
                        \ joystick key