Version analysis of LL9 (Part 12 of 12)

Cassette, Flight, Docked, 6502SP, Electron
Master
 This part draws the lines in the ship line heap, which is used both to draw
 the ship, and to remove it from the screen.
 This part draws any remaining lines from the old ship that are still in the
 ship line heap.

6502SP
 If NEEDKEY is non-zero, then this routine also scans the keyboard for a key
 press and returns the internal key number in X (or 0 for no key press).

Master
 Other entry points:

   LSCLR                Draw any remaining lines from the old ship that are
                        still in the ship line heap

   LSC3                 Contains an RTS

Cassette, Flight, Docked, 6502SP, Electron
Master
.LL155
.LSCLR

6502SP
 LDA NEEDKEY            \ If NEEDKEY is zero, jump to notneed to skip the next
 BEQ notneed            \ two instructions, so we only read the keyboard if
                        \ NEEDKEY is non-zero

 STZ NEEDKEY            \ Set NEEDKEY = 0

 JSR RDKEY              \ Scan the keyboard for a key press and return the
                        \ internal key number in X (or 0 for no key press)

.notneed

Cassette, Flight, Docked, Electron
6502SP
 LDY #0                 \ Fetch the first byte from the ship line heap into A,
 LDA (XX19),Y           \ which contains the number of bytes in the heap

 STA XX20               \ Store the heap size in XX20

 CMP #4                 \ If the heap size is less than 4, there is nothing to
 BCC LL118-1            \ draw, so return from the subroutine (as LL118-1
                        \ contains an RTS)
 LDA (XX19)             \ Fetch the first byte from the ship line heap into A,
                        \ which contains the number of bytes in the heap

 CMP #5                 \ If the heap size is less than 5, there is nothing to
 BCC nolines            \ draw, so return from the subroutine (as nolines
                        \ contains an RTS)

 LDA #129               \ Send an OSWRCH 129 command to the I/O processor to
 JSR OSWRCH             \ tell it to start receiving a new line to draw (so
                        \ when we send it OSWRCH commands from now on, the I/O
                        \ processor will add these bytes to this line until
                        \ they are all sent, at which point it will draw the
                        \ line)

Cassette, Flight, Docked, Electron
Master
6502SP
 INY                    \ Set Y = 1, which we will use as an index into the ship
                        \ line heap, starting at byte #1 (as byte #0 contains
                        \ the heap size)
 LDY LSNUM              \ Set Y to the offset in the line heap LSNUM
 LDY #0                 \ Fetch the first byte from the ship line heap into A,
 LDA (XX19),Y           \ which contains the number of bytes in the heap

 STA XX20               \ Store the heap size in XX20

Cassette, Flight, Docked, Electron
Master
6502SP
.LL27

 LDA (XX19),Y           \ Fetch the X1 line coordinate from the heap and store
 STA XX15               \ it in XX15

 INY                    \ Increment the heap pointer

 LDA (XX19),Y           \ Fetch the Y1 line coordinate from the heap and store
 STA XX15+1             \ it in XX15+1

 INY                    \ Increment the heap pointer

 LDA (XX19),Y           \ Fetch the X2 line coordinate from the heap and store
 STA XX15+2             \ it in XX15+2

 INY                    \ Increment the heap pointer

 LDA (XX19),Y           \ Fetch the Y2 line coordinate from the heap and store
 STA XX15+3             \ it in XX15+3

 JSR LL30               \ Draw a line from (X1, Y1) to (X2, Y2)
.LSC1

 CPY LSNUM2             \ If Y >= LSNUM2, jump to LSC2 to return from the ship
 BCS LSC2               \ drawing routine, because the index in Y is greater
                        \ than the size of the existing ship line heap, which
                        \ means we have already erased all the old ship's lines
                        \ when drawing the new ship

                        \ If we get here then Y < LSNUM2, which means Y is
                        \ pointing to an on-screen line from the old ship that
                        \ we need to erase

 LDA (XX19),Y           \ Fetch the X1 line coordinate from the heap and store
 INY                    \ it in XX15, incrementing the heap pointer
 STA XX15

 LDA (XX19),Y           \ Fetch the Y1 line coordinate from the heap and store
 INY                    \ it in XX15+1, incrementing the heap pointer
 STA XX15+1

 LDA (XX19),Y           \ Fetch the X2 line coordinate from the heap and store
 INY                    \ it in XX15+2, incrementing the heap pointer
 STA XX15+2

 LDA (XX19),Y           \ Fetch the Y2 line coordinate from the heap and store
 INY                    \ it in XX15+3, incrementing the heap pointer
 STA XX15+3

 JSR LOIN               \ Draw a line from (X1, Y1) to (X2, Y2) to erase it from
                        \ the screen
.LL27

 LDA (XX19),Y           \ Fetch the Y-th line coordinate from the heap and send
 JSR OSWRCH             \ it to the I/O processor to add to the line buffer

Cassette, Flight, Docked, 6502SP, Electron
Master
 INY                    \ Increment the heap pointer

 CPY XX20               \ If the heap counter is less than the size of the heap,
 BCC LL27               \ loop back to LL27 to draw the next line from the heap
 JMP LSC1               \ Loop back to LSC1 to draw (i.e. erase) the next line
                        \ from the heap

.LSC2

 LDA LSNUM              \ Store LSNUM in the first byte of the ship line heap
 LDY #0
 STA (XX19),Y

.LSC3

Cassette
6502SP
\.LL82                  \ This label is commented out in the original source
                        \ By the time we get here, we have sent all the line
                        \ coordinates to the I/O processor, so it will have
                        \ drawn the line after we sent the last one

.nolines