SymbolShift Posted October 8, 2025 Report Share Posted October 8, 2025 4 hours ago, Norman Sword said: See "altered reality" for corrected Willy movement. And it does not feel strange. Although I'm not against using corrected mechanics, I would be unsure on how to modify my code to make the changes. Quote Link to comment Share on other sites More sharing options...
JianYang Posted October 11, 2025 Report Share Posted October 11, 2025 On 10/7/2025 at 9:07 PM, SymbolShift said: Should I use the original mechanics (warts and all) to be faithful to the originals? MM mechanics for a MM clone, and JSW mechanics for a JSW clone, or new mechanics altogether? My concern is that someone unfamiliar with the original mechanics quirks, could play my game and interpret them as bad programming. The superstar solution would be to give the game an "enable bug compatibility" option. I'm not sure what platform you're talking about. On a speccy 48k that would probably require wizard levels of programming on a more modern platform it would still be hard. Just throwing that out in case it wasn't obvious. jetsetdanny and SymbolShift 2 Quote Link to comment Share on other sites More sharing options...
fawtytoo Posted October 16, 2025 Report Share Posted October 16, 2025 This bit took me ages to work out using the disassembly from skoolkit. On 10/7/2025 at 8:07 PM, SymbolShift said: It appears that Matthew Smith's solution was to simply try and avoid those situations, rather than rewrite the movement mechanics. As regards JSW, the movement mechanics were altered to fit the situations (such as The Forgotten Abbey), hence the head height bug. In my re-write, I fixed the situations with new block types to keep the MM mechanics. Of course, I did the changes only to fix the bugs and not change the game play. Quote Link to comment Share on other sites More sharing options...
SymbolShift Posted October 16, 2025 Report Share Posted October 16, 2025 1 hour ago, fawtytoo said: This bit took me ages to work out using the disassembly from skoolkit. As regards JSW, the movement mechanics were altered to fit the situations (such as The Forgotten Abbey), hence the head height bug. In my re-write, I fixed the situations with new block types to keep the MM mechanics. Of course, I did the changes only to fix the bugs and not change the game play. Right, MS altered the left movement collision intentionally to allow for certain situations, but side-effects were created as a result of the modification, which were then mostly avoided. Your solution with new block types works very well! Quote Link to comment Share on other sites More sharing options...
Norman Sword Posted October 18, 2025 Report Share Posted October 18, 2025 (edited) A big post of z80 code The reason for the left right anomaly is simply caused by poor code layout as done by Matthew. When setting up movement code - it is normal to mirror the code for left and right. This simple procedure allows quick and accurate visual comparisons. Here the first lot of code is the layout of checking left and right movement when possibly on a ramp NOTE this has unused opcodes and does not mirror left and right Expect visual changes - over time on what you can see here. I will probably edit the visual layout. And correct errors as I see them. MATTHEWS CODE:- Used by the routines at 8DD3 and 8ED4. This routine moves Willy left or right if necessary. 8FBC LD A,($85D0) Pick up Willy's direction and movement flags from 85D0 8FBF AND $02 Is Willy moving left or right? 8FC1 RET Z Return if not 8FC2 LD A,($85D6) Pick up the rope status indicator from 85D6 8FC5 DEC A Is Willy on a rope? 8FC6 BIT 7,A 8FC8 RET Z Return if so (Willy's movement along a rope is handled at 935E) 8FC9 LD A,($85D0) Pick up Willy's direction and movement flags from 85D0 8FCC AND $01 Is Willy facing right? 8FCE JP Z,$9042 Jump if so ; LEFT AND RIGHT CODE - DISPLAYED NEXT TO EACH OTHER Willy is moving right. ; Willy is moving left. 9042 LD A,($85D2) ; 8FD1 LD A,($85D2) Willy's animation frame from 85D2 9045 CP $03 ; 8FD4 OR A Is it 0? 9047 JR Z,$904E ; 8FD5 JR Z,$8FDC If so, jump to move Willy's sprite left across a cell boundary 9049 INC A ; 8FD7 DEC A Change Willy's animation frame at 85D2 904A LD ($85D2),A ;8FD8 LD ($85D2),A 904D RET ; 8FDB RET Willy's sprite is moving right ; Willy's sprite is moving left across a cell boundary. In the comments that follow, (x,y) refers to the coordinates of the top-left cell currently occupied by Willy's sprite. 904E LD A,($85D1) ; 8FDC LD A,($85D1) Pick up the airborne status indicator from 85D1 9051 LD BC,$0000 ; 8FDF LD BC,$0000 Prepare BC for later addition 9054 OR A ; 8FE2 CP $00 Is Willy jumping? 9055 JR NZ,$9078 ; 8FE4 JR NZ,$900A Jump if so 9057 LD HL,($85D3) ; 8FE6 LD HL,($85D3) Collect Willy's attribute buffer coordinates from 85D3 ; 8FE9 LD BC,$0000 Prepare BC for later addition (again, redundantly) 905A LD A,($80DA) ; 8FEC LD A,($80DA) Pick up the direction byte of the ramp definition for the current room from 80DA 905D DEC A ; 8FEF DEC A Now A=0x1F if the ramp goes up to the left, or 0x41 if it goes up to the right 905E OR $9D ; 8FF0 OR $A1 9060 XOR $BF ; 8FF2 XOR $E0 9062 LD E,A ; 8FF4 LD E,A Point HL at the cell at (x-1,y+1) if the ramp goes up to the left, or at the cell at (x+1,y+2) if the ramp goes up to the right 9063 LD D,$00 ; 8FF5 LD D,$00 9065 ADD HL,DE ; 8FF7 ADD HL,DE 9066 LD A,($80C4) ; 8FF8 LD A,($80C4) Pick up the attribute byte of the ramp tile for the current room from 80C4 9069 CP (HL) ; 8FFB CP (HL) Is there a ramp tile in the cell pointed to by HL? 906A JR NZ,$9078 ; 8FFC JR NZ,$900A Jump if not 906C LD BC,$0020 ; 8FFE LD BC,$0020 Prepare BC for later addition 906F LD A,($80DA) ; 9001 LD A,($80DA) Pick up the direction byte of the ramp definition for the current room from 80DA 9072 OR A ; 9004 OR A Does the ramp go up to the right? 9073 JR Z,$9078 ; 9005 JR NZ,$900A Jump if so 9075 LD BC,$FFE0 ; 9007 LD BC,$FFE0 BC=-32 (the ramp goes up to the left) 9078 LD HL,($85D3) ; 900A LD HL,($85D3) Collect Willy's attribute buffer coordinates from 85D3 ; check extremes far right ; and far left - jump room 907B ADD HL,BC ; 907C INC HL ; 907D INC HL ; 907E LD A,L ; 900D LD A,L Is Willy's screen x-coordinate 0 (on the far left)? 907F AND $1F ; 900E AND $1F 9081 JP Z,$949E ; 9010 JP Z,$948A If so, move Willy into the room to the left; 9084 LD DE,$0020 ; 9087 LD A,($80B2) ; ; 9013 ADD HL,BC Point HL at the cell at (x-1,y+1), or at the cell at (x-1,y) if Willy is on or about to step onto a ramp that goes up to the left, or at the cell at (x-1,y+2) if Willy is walking down a ramp ; 9014 DEC HL ; 9015 LD DE,$0020 908A ADD HL,DE ; 9018 ADD HL,DE ; 9019 LD A,($80B2) Pick up the attribute byte of the wall tile for the current room from 80B2 908B CP (HL) ; 901C CP (HL) Is there a wall tile in the cell pointed to by HL? 908C RET Z ; 901D RET Z Return if so without moving Willy (his path is blocked) 908D LD A,($85CF) ; 901E LD A,($85CF) Pick up Willy's y-coordinate (Y) from 85CF 9090 SRA C ; 9021 SRA C Now B=Y (if Willy is neither on nor about to step onto a ramp), or Y+16 (if Willy is walking down a ramp), or Y-16 (if Willy is on or about to step onto a ramp that goes up to the left) this will be Willy's new y-coordinate 9092 ADD A,C ; 9023 ADD A,C 9093 LD B,A ; 9024 LD B,A 9094 AND $0F ; 9025 AND $0F Is Willy at a point in a jump (left) where his sprite occupies three rows of cells? 9096 JR Z,$90A1 ; 9027 JR Z,$9032 Jump if not (Willy's sprite is cell-aligned) 9098 LD A,($80B2) ; 9029 LD A,($80B2) Pick up the attribute byte of the wall tile for the current room from 80B2 909B ADD HL,DE ; 902C ADD HL,DE Point HL at the cell at (x-1,y+2) 909C CP (HL) ; 902D CP (HL) Is there a wall tile there? 909D RET Z ; 902E RET Z Return if so without moving Willy (his path is blocked) 909E OR A ; 902F OR A Point HL at the cell at (x-1,y+1) 909F SBC HL,DE ; 9030 SBC HL,DE 90A1 LD A,($80B2) ; 90A4 OR A ; 9032 OR A Point HL at the cell at (x-1,y), or at the cell at (x-1,y-1) if Willy is on or about to step onto a ramp that goes up to the left, or at the cell at (x-1,y+1) if Willy is walking down a ramp 90A5 SBC HL,DE ; 9033 SBC HL,DE 90A7 CP (HL) ; 90A8 RET Z ; 90A9 DEC HL ; 90AA LD ($85D3),HL ; 9035 LD ($85D3),HL Save Willy's new attribute buffer coordinates (in HL) at 85D3 90AD XOR A 90AE LD ($85D2),A ; 90B1 LD A,B ; 9038 LD A,B Save Willy's new y-coordinate at 85CF ; 90B2 LD ($85CF),A ; 9039 LD ($85CF),A 90B5 RET ; 903C LD A,$03 Change Willy's animation frame at 85D2 from 0 to 3 ; 903E LD ($85D2),A ; 9041 RET ;============================================= The above code laid out with symatry - NOTE the bug can be inserted by just removing the head check. Here I relay out the code with symetry Used by the routines at 8DD3 and 8ED4. This routine moves Willy left or right if necessary. 8FC2: LD A,($85D6) Pick up the rope status indicator from 85D6 DEC A Is Willy on a rope? RET m Return if so (Willy's movement along a rope is handled at 935E) LD A,($85D0) Pick up Willy's direction and movement flags from 85D0 bit 1,a Is Willy moving left or right? RET Z Return if not bit 0,a Is Willy facing right? ;set up some data for later LD A,($85D2) Pick up Willy's animation frame from 85D2 LD BC,$0000 set no displacement in bc JP Z,rampR Jump if facing right Code would drop through to rampL Willy is moving right. ; Willy is moving left. rampR: ;rampL CP $03 OR A Is it 0? JR Z,right ; JR Z,left If so, jump to move Willy's sprite left across a cell boundary INC A ; DEC A Decrement Willy's animation frame at 85D2 LD ($85D2),A ; LD ($85D2),A RET ; RET Willy's sprite is moving right ; Willy's sprite is moving left across a cell boundary. In the comments that follow, (x,y) refers to the coordinates of the top-left cell currently occupied by Willy's sprite. right: ; left LD A,($85D1) LD A,($85D1) Pick up the airborne status indicator from 85D1 OR A ; OR A Is Willy jumping? JR NZ,nrampR ; JR NZ,nrampL Jump if so - NO RAMP ADJUSTMENT LD HL,($85D3) ; LD HL,($85D3) Collect Willy's attribute buffer coordinates from 85D3 LD A,($80DA) ; LD A,($80DA) Pick up the direction byte of the ramp definition for the current room from 80DA DEC A ; DEC A Now A=0x1F if the ramp goes up to the left, or 0x41 if it goes up to the right OR $9D ; OR $A1 XOR $BF ; XOR $E0 LD E,A ; LD E,A Point HL at the cell at (x-1,y+1) if the ramp goes up to the left, or at the cell at (x+1,y+2) if the ramp goes up to the right LD D,$00 ; LD D,$00 ADD HL,DE ; ADD HL,DE ; is the ramp undefoot ? LD A,($80C4) ; LD A,($80C4) Pick up the attribute byte of the ramp tile for the current room from 80C4 CP (HL) ; CP (HL) Is there a ramp tile in the cell pointed to by HL? JR NZ,nrampR ; JR NZ,nrampL Jump if not ; set bc for ramp downwards LD BC,$0020 ; LD BC,$0020 Prepare BC for later addition LD A,($80DA) ; LD A,($80DA) Pick up the direction byte of the ramp definition for the current room from 80DA OR A ; OR A Does the ramp go up to the right? JR Z,nrampR ; JR NZ,nrampL Jump if so ; set bc for ramp upwards LD BC,$FFE0 ; LD BC,$FFE0 BC=-32 (the ramp goes up to the left) ; before going any further check for going off screen nrampR: ;nrampL: LD HL,($85D3) ; LD HL,($85D3) Collect Willy's attribute buffer coordinates from 85D3 ; position adjust INC HL ; INC HL ; LD A,L ; LD A,L Is Willy's screen x-coordinate 0 (on the far left)? AND $1F ; AND $1F JP Z,$949E ; JP Z,$948A If so, move Willy into the room to the left; ; the above jumped room at the left/right edge ; adjust position if on a ramp - i.e move up/down - note bc=0 if not on a ramp add hl,bc ; ADD HL,BC this is the ramp adjustment ; re align the checking for walls ; DEC HL ld de,$0020 ; LD DE,$0020 Displacement for the tile below ld a,($80b2) ; LD A,($80B2) Pick up the attribute byte of the wall tile for the current room from 80B2 LD B,A ; LD B,A save wall tile value in b ; head check - to implement bug - remove the check - can do either left or right - perhaps both CP (HL) ; CP (HL) Is there a wall tile in the cell pointed to by HL? RET Z ; RET Z Return if so without moving Willy (his path is blocked) ; BODY CHECK ADD HL,DE ; ADD HL,DE CP (HL) ; CP (HL) RET Z ; RET Z ;calc new y position LD A,($85CF) ; LD A,($85CF) Pick up Willy's y-coordinate (Y) from 85CF SRA C ; SRA C Now B=Y (if Willy is neither on nor about to step onto a ramp), or Y+16 (if Willy is walking down a ramp), or Y-16 (if Willy is on or about to step onto a ramp that goes up to the left); this will be Willy's new y-coordinate ADD A,C ; ADD A,C LD c,A ; LD c,A ; now check if cell aligned AND $0F ; AND $0F Is Willy at a point in a jump (left) where his sprite occupies three rows of cells? JR Z,ramp3R ; JR Z,ramp3L Jump if not (Willy's sprite is cell-aligned) ; if not aligned check 3rd tile LD A,B ; LD A,B ADD HL,DE ; ADD HL,DE Point HL at the cell at (x-1,y+2) CP (HL) ; CP (HL) Is there a wall tile there? RET Z ; RET Z Return if so without moving Willy (his path is blocked) OR A ; OR A Point HL at the cell at (x-1,y+1) SBC HL,DE ; SBC HL,DE restore back to position before 3rd tile was checked ; adjust position back to level with the top tile ramp3R: ;ramp3L: OR A ; OR A SBC HL,DE ; SBC HL,DE ; ; and re REALIGN for top left DEC HL ; ; save data and exit LD ($85D3),HL ; LD ($85D3),HL Save Willy's new attribute buffer coordinates (in HL) at 85D3 ld a,c ; LD A,c Get Willy's new y-coordinate LD ($85CF),A ; LD ($85CF),A Save Willy's new y-coordinate at 85CF xor a ; LD A,$03 Change Willy's animation frame at 85D2 from 0 to 3 ld ($85d2),a ; LD ($85D2),A RET ; RET ;============================================== The Feature that is displayed in the movement code. When it was implemented it allowed a feature and that feature was used. It is similar to the pause bug, attic arrow bug, missing and uncollectable objects etc Whilst I think my code is correct - if I notice errors - then I will correct them. The purpose of the code is to show code being laid out as similar for both directions. ; Edited October 19, 2025 by Norman Sword Improving layout - tabs do not translate. Jet Set Willie, jetsetdanny, CPL and 2 others 1 4 Quote Link to comment Share on other sites More sharing options...
Ant Posted January 31 Report Share Posted January 31 This is a really good write-up - Wish I'd seen this before I went through the pain of working it out for myself. I took a slightly different approach that feels 100% authentic while keeping Willy's coordinate centre within the frame. The one problem I encountered was jumping diagonally through a 2-cell gap next to a wall - e.g. out of the portal and to the right on Kong Beast. If the Y overshot the gap when jumping, the X head check would detect the block above and prevent Willy from seeing the gap. Fixed it with some edge case code, but I would have liked a tidier solution. Didn't help that I'm using floating point. Quote Link to comment Share on other sites More sharing options...
fawtytoo Posted February 1 Report Share Posted February 1 16 hours ago, Ant said: Didn't help that I'm using floating point. Any particular reason you chose floating point? Quote Link to comment Share on other sites More sharing options...
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