Version analysis of HAS2 - Elite on the BBC Micro and NES

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

Disc (docked), 6502 Second Processor and Master

Code variations between these versions are shown below.

       Name: HAS2
       Type: Subroutine
   Category: Ship hangar
    Summary: Draw a hangar background line from left to right


 This routine draws a line to the right, starting with the third pixel of the
 pixel row at screen address SC(1 0), and aborting if we bump into something
 that's already on-screen.

Code variation 1 of 6A variation in the comments only

This variation is blank in the Disc (docked) version.

6502SP, Master
 HAL2 draws from the left edge of the screen to the halfway point, and then
 HAL3 takes over to draw from the halfway point across the right half of the
 screen.

Code variation 2 of 6A variation in the comments only

This variation is blank in the 6502 Second Processor and Master versions.

Docked
 Other entry points:

   HA3                  Contains an RTS


.HAS2

Code variation 3 of 6Related to the screen mode

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

Docked
6502SP, Master
 LDA #%00100000         \ Set A to the pixel pattern for a mode 4 character row
                        \ byte with the third pixel set, so we start drawing the
                        \ horizontal line just to the right of the 2-pixel
                        \ border along the edge of the screen
 LDA #%00100010         \ Set A to the pixel pattern for a mode 1 character row
                        \ byte with the third pixel set, so we start drawing the
                        \ horizontal line just to the right of the 2-pixel
                        \ border along the edge of the screen

.HAL2

 TAX                    \ Store A in X so we can retrieve it after the following
                        \ check and again after updating screen memory

 AND (SC),Y             \ If the pixel we want to draw is non-zero (using A as a
 BNE HA3                \ mask), then this means it already contains something,
                        \ so we stop drawing because we have run into something
                        \ that's already on-screen, and return from the
                        \ subroutine (as HA3 contains an RTS)

 TXA                    \ Retrieve the value of A we stored above, so A now
                        \ contains the pixel mask again

Code variation 4 of 6Related to the screen mode

This variation is blank in the Disc (docked) version.

6502SP, Master
 AND #RED               \ Apply the pixel mask in A to a four-pixel block of
                        \ red pixels, so we now know which bits to set in screen
                        \ memory

 ORA (SC),Y             \ OR the byte with the current contents of screen
                        \ memory, so the pixel we want is set to red (because
                        \ we know the bits are already 0 from the above test)

 STA (SC),Y             \ Store the updated pixel in screen memory

 TXA                    \ Retrieve the value of A we stored above, so A now
                        \ contains the pixel mask again

 LSR A                  \ Shift A to the right to move on to the next pixel

 BCC HAL2               \ If bit 0 before the shift was clear (i.e. we didn't
                        \ just do the fourth pixel in this block), loop back to
                        \ HAL2 to check and draw the next pixel

 TYA                    \ Set Y = Y + 8 (as we know the C flag is set) to point
 ADC #7                 \ to the next character block along
 TAY

Code variation 5 of 6Related to the screen mode

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

Docked
6502SP, Master
 LDA #%10000000         \ Reset the pixel mask in A to the first pixel in the
                        \ new 8-pixel character block
 LDA #%10001000         \ Reset the pixel mask in A to the first pixel in the
                        \ new 4-pixel character block

 BCC HAL2               \ If the above addition didn't overflow, jump back to
                        \ HAL2 to keep drawing the line in the next character
                        \ block

Code variation 6 of 6Related to the screen mode

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

Docked
6502SP, Master
.HA3

 RTS                    \ The addition overflowed, so we have reached the last
                        \ character block in this page of memory, which is the
                        \ end of the line, so we return from the subroutine
 INC SC+1               \ The addition overflowed, so we have reached the last
                        \ character block in this page of memory, so increment
                        \ the high byte of SC(1 0) in SC+1 to point to the next
                        \ page (i.e. the right half of this screen row) and fall
                        \ into HAL3 to repeat the performance