429 lines
16 KiB
NASM
429 lines
16 KiB
NASM
\ ******************************************************************************
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\
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\ BBC MASTER ELITE GAME SOURCE (FLICKER-FREE ROUTINES)
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\
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\ BBC Master Elite was written by Ian Bell and David Braben and is copyright
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\ Acornsoft 1986
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\
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\ The code on this site has been reconstructed from a disassembly of the version
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\ released on Ian Bell's personal website at http://www.elitehomepage.org/
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\
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\ The commentary is copyright Mark Moxon, and any misunderstandings or mistakes
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\ in the documentation are entirely my fault
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\
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\ The terminology and notations used in this commentary are explained at
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\ https://www.bbcelite.com/about_site/terminology_used_in_this_commentary.html
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\
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\ The deep dive articles referred to in this commentary can be found at
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\ https://www.bbcelite.com/deep_dives
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\
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\ ******************************************************************************
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GUARD &CE00 \ Guard against assembling over memory used by game
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\ ******************************************************************************
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\
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\ Configuration variables
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\
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\ The addresses in the following are from when the game binary is loaded into
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\ memory. They were calculated by analysing a memory dump of the running game,
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\ searching for patterns in the bytes to match them with the corrsponding code
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\ from the BBC Micro version (which is very similar, if you ignore any different
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\ addresses).
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\
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\ XX14 is an unused variable in BBC Micro Elite, and the corresponding address
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\ in Commodore 64 Elite is used by something else. Luckily locations $FB to $FE
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\ are unused by Elite and the OS, so we can sneak XX14 in there instead.
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\
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\ Also, the Y variable contains the height of the space view in pixels, divided
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\ by 2. This is 96 on the BBC Micro, but the space view is 144 pixels on the
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\ Commodore 64, so Y needs to be set to 72 instead.
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\
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\ ******************************************************************************
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XX1 = $0009
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INWK = $0009
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XX19 = $002A
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CNT = $0030
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K3 = $0035
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K4 = $0043
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XX0 = $0057
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V = $005B
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XX15 = $006B
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X1 = $006B
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Y1 = $006C
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X2 = $006D
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Y2 = $006E
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XX12 = $0071
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XX17 = $009F
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XX4 = $00AD
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XX20 = $00AE
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XX14 = $00FB
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PROJ = $7D1F
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LL75 = $9FB8
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LL80 = $A13F
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LL30 = $AB91
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Y = 72
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\ ******************************************************************************
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\
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\ Name: SHPPT
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\ Type: Subroutine
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\ Category: Drawing ships
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\ Summary: Draw a distant ship as a point rather than a full wireframe
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\
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\ ******************************************************************************
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ORG $9932
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.SHPPT
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JSR PROJ \ Project the ship onto the screen, returning:
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\
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\ * K3(1 0) = the screen x-coordinate
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\ * K4(1 0) = the screen y-coordinate
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\ * A = K4+1
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ORA K3+1 \ If either of the high bytes of the screen coordinates
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BNE nono \ are non-zero, jump to nono as the ship is off-screen
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LDA K4 \ Set A = the y-coordinate of the dot
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CMP #Y*2-2 \ If the y-coordinate is bigger than the y-coordinate of
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BCS nono \ the bottom of the screen, jump to nono as the ship's
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\ dot is off the bottom of the space view
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JSR Shpt \ Call Shpt to draw a horizontal 4-pixel dash for the
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\ first row of the dot (i.e. a four-pixel dash)
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LDA K4 \ Set A = y-coordinate of dot + 1 (so this is the second
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CLC \ row of the two-pixel-high dot)
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ADC #1
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JSR Shpt \ Call Shpt to draw a horizontal 4-pixel dash for the
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\ first row of the dot (i.e. a four-pixel dash)
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LDA #%00001000 \ Set bit 3 of the ship's byte #31 to record that we
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ORA XX1+31 \ have now drawn something on-screen for this ship
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STA XX1+31
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JMP LL155 \ Jump to LL155 to draw any remaining lines that are
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\ still in the ship line heap and return from the
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\ subroutine using a tail call
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.nono
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LDA #%11110111 \ Clear bit 3 of the ship's byte #31 to record that
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AND XX1+31 \ nothing is being drawn on-screen for this ship
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STA XX1+31
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JMP LL155 \ Jump to LL155 to draw any remaining lines that are
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\ still in the ship line heap and return from the
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\ subroutine using a tail call
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.Shpt
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\ This routine draws a horizontal 4-pixel dash, for
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\ either the top or the bottom of the ship's dot
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STA Y1 \ Store A in both y-coordinates, as this is a horizontal
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STA Y2 \ dash at y-coordinate A
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LDA K3 \ Set A = screen x-coordinate of the ship dot
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STA X1 \ Store the x-coordinate of the ship dot in X1, as this
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\ is where the dash starts
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CLC \ Set A = screen x-coordinate of the ship dot + 3
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ADC #3
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BCC P%+4 \ If the addition overflowed, set A = 255, the
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LDA #255 \ x-coordinate of the right edge of the screen
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STA X2 \ Store the x-coordinate of the ship dot in X1, as this
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\ is where the dash starts
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JMP LLX30 \ Draw this edge using flicker-free animation, by first
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\ drawing the ship's new line and then erasing the
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\ corresponding old line from the screen, and return
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\ from the subroutine using a tail call
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SAVE "shppt.bin", SHPPT, P%
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\ ******************************************************************************
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\
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\ Name: LL9 (Part 11 of 12)
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\ Type: Subroutine
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\ Category: Drawing ships
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\ Summary: Draw ship: Loop back for the next edge
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\ Deep dive: Drawing ships
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\
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\ ******************************************************************************
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ORG $A15B
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.LL78
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LDA XX14 \ If XX14 >= CNT, skip to LL81 so we don't loop back for
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CMP CNT \ the next edge (CNT was set to the maximum heap size
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BCS LL81 \ for this ship in part 10, so this checks whether we
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\ have just run out of space in the ship line heap, and
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\ stops drawing edges if we have)
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LDA V \ Increment V by 4 so V(1 0) points to the data for the
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CLC \ next edge
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ADC #4
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STA V
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BCC ll81 \ If the above addition didn't overflow, jump to ll81
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INC V+1 \ Otherwise increment the high byte of V(1 0), as we
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\ just moved the V(1 0) pointer past a page boundary
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.ll81
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INC XX17 \ Increment the edge counter to point to the next edge
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LDY XX17 \ If Y >= XX20, which contains the number of edges in
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CPY XX20 \ the blueprint, skip the following
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BCS P%+5
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JMP LL75-2 \ Loop back to LL75-2 to process the next edge (we jump
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\ to LL75-2 as we have modified the code around LL75,
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\ which moves the jump point back by two bytes)
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.LL81
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NOP
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SAVE "ll78.bin", LL78, P%
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\ ******************************************************************************
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\
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\ Name: LL9 (Part 12 of 12)
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\ Type: Subroutine
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\ Category: Drawing ships
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\ Summary: Draw ship: Draw all the visible edges from the ship line heap
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\ Deep dive: Drawing ships
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\
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\ ******************************************************************************
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ORG $A178
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.LL155
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LDY XX14 \ Set Y to the offset in the line heap XX14
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.LL27
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CPY XX14+1 \ If Y >= XX14+1, jump to LLEX to return from the ship
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BCS LLEX \ drawing routine, because the index in Y is greater
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\ than the size of the existing ship line heap, which
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\ means we have alrady erased all the old ships lines
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\ when drawing the new ship
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\ If we get here then Y < XX14+1, which means Y is
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\ pointing to an on-screen line from the old ship that
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\ we need to erase
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LDA (XX19),Y \ Fetch the X1 line coordinate from the heap and store
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STA XX15 \ it in XX15
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INY \ Increment the heap pointer
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LDA (XX19),Y \ Fetch the Y1 line coordinate from the heap and store
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STA XX15+1 \ it in XX15+1
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INY \ Increment the heap pointer
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LDA (XX19),Y \ Fetch the X2 line coordinate from the heap and store
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STA XX15+2 \ it in XX15+2
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INY \ Increment the heap pointer
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LDA (XX19),Y \ Fetch the Y2 line coordinate from the heap and store
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STA XX15+3 \ it in XX15+3
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JSR LL30 \ Draw a line from (X1, Y1) to (X2, Y2) to erase it from
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\ the screen
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INY \ Increment the heap pointer
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JMP LL27 \ Loop back to LL27 to draw (i.e. erase) the next line
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\ from the heap
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.LLEX
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LDA XX14 \ Store XX14 in the first byte of the ship line heap
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LDY #0
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STA (XX19),Y
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.LL82
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RTS \ Return from the subroutine
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SAVE "ll155.bin", LL155, P%
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ORG $CCE0
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\ ******************************************************************************
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\
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\ Name: LLX30
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\ Type: Subroutine
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\ Category: Drawing lines
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\ Summary: Draw a ship line using flicker-free animation
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\
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\ ******************************************************************************
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.LLX30
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LDY XX14 \ Set Y = XX14, to get the offset within the ship line
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\ heap where we want to insert our new line
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CPY XX14+1 \ Compare XX14 and XX14+1 and store the flags on the
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PHP \ stack so we can retrieve them later
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LDX #3 \ We now want to copy the line coordinates (X1, Y1) and
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\ (X2, Y2) to XX12...XX12+3, so set a counter to copy
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\ 4 bytes
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.LLXL
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LDA X1,X \ Copy the X-th byte of X1/Y1/X2/Y2 to the X-th byte of
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STA XX12,X \ XX12
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DEX \ Decrement the loop counter
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BPL LLXL \ Loop back until we have copied all four bytes
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JSR LL30 \ Draw a line from (X1, Y1) to (X2, Y2)
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LDA (XX19),Y \ Set X1 to the Y-th coordinate on the ship line heap,
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STA X1 \ i.e. one we are replacing in the heap
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LDA XX12 \ Replace it with the X1 coordinate in XX12
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STA (XX19),Y
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INY \ Increment the index to point to the Y1 coordinate
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LDA (XX19),Y \ Set Y1 to the Y-th coordinate on the ship line heap,
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STA Y1 \ i.e. one we are replacing in the heap
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LDA XX12+1 \ Replace it with the Y1 coordinate in XX12+1
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STA (XX19),Y
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INY \ Increment the index to point to the X2 coordinate
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LDA (XX19),Y \ Set X1 to the Y-th coordinate on the ship line heap,
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STA X2
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LDA XX12+2 \ Replace it with the X2 coordinate in XX12+2
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STA (XX19),Y
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INY \ Increment the index to point to the Y2 coordinate
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LDA (XX19),Y \ Set Y2 to the Y-th coordinate on the ship line heap,
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STA Y2
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LDA XX12+3 \ Replace it with the Y2 coordinate in XX12+3
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STA (XX19),Y
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INY \ Increment the index to point to the next coordinate
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STY XX14 \ and store the updated index in XX14
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PLP \ Restore the result of the comparison above, so if the
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BCS LL82a \ original value of XX14 >= XX14+1, then we have already
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\ redrawn all the lines from the old ship's line heap,
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\ so return from the subroutine (as LL82 contains an
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\ RTS)
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JMP LL30 \ Otherwise there are still more lines to erase from the
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\ old ship on-screen, so the coordinates in (X1, Y1) and
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\ (X2, Y2) that we just pulled from the ship line heap
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\ point to a line that is still on-screen, so call LL30
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\ to draw this line and erase it from the screen,
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\ returning from the subroutine using a tail call
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.LL82a
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RTS \ Return from the subroutine
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\ ******************************************************************************
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\
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\ Name: LL9 (Part 1 of 12)
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\ Type: Subroutine
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\ Category: Drawing ships
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\ Summary: Draw ship: Check if ship is exploding, check if ship is in front
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\ Deep dive: Drawing ships
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\
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\ ******************************************************************************
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.PATCH1
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\ We replace the following two instructions in part 1 of
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\ LL9 with JSR PATCH1, so we start with those two
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\ instructions to ensure that they still get done
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LDA #31 \ Set XX4 = 31 to store the ship's distance for later
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STA XX4 \ comparison with the visibility distance. We will
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\ update this value below with the actual ship's
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\ distance if it turns out to be visible on-screen
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\ We now set things up for flicker-free ship plotting,
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\ by setting the following:
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\
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\ XX14 = offset to the first coordinate in the ship's
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\ line heap
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\
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\ XX14+1 = the number of bytes in the heap for the
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\ ship that's currently on-screen (or 0 if
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\ there is no ship currently on-screen)
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LDY #1 \ Set XX14 = 1, the offset of the first set of line
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STY XX14 \ coordinates in the ship line heap
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DEY \ Decrement Y to 0
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LDA #%00001000 \ If bit 3 of the ship's byte #31 is set, then the ship
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BIT INWK+31 \ is currently being drawn on-screen, so skip the
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BNE P%+5 \ following two instructions
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LDA #0 \ The ship is not being drawn on screen, so set A = 0
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\ so that XX14+1 gets set to 0 below (as there are no
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\ existing coordinates on the ship line heap for this
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\ ship)
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EQUB $2C \ Skip the next instruction by turning it into
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\ $2C $B1 $BD, or BIT $BDB1 which does nothing apart
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\ from affect the flags
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LDA (XX19),Y \ Set XX14+1 to the first byte of the ship's line heap,
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STA XX14+1 \ which contains the number of bytes in the heap
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RTS
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\ ******************************************************************************
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\
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\ Name: LL9 (Part 10 of 12)
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\ Type: Subroutine
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\ Category: Drawing ships
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\ Summary: Draw ship: Check if ship is exploding, check if ship is in front
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\ Deep dive: Drawing ships
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\
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\ ******************************************************************************
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.PATCH2
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\ We replace the JMP LL78 instruction at the end of part
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\ 10 of LL9 with JSR PATCH2, so this effectively inserts
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\ the call to LLX30 at the end of part 10, as required
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JSR LLX30 \ Draw the laser line using flicker-free animation, by
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\ first drawing the new laser line and then erasing the
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\ corresponding old line from the screen
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JMP LL78 \ Jump down to part 11
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SAVE "extra.bin", LLX30, P%
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