Toledo Atomchess Game

• Plays basic chess movements (no en passant, no castling and no promotion)
• Enter your movements as basic algebraic (D2D4)
• Your movements aren't checked for legality
• Search depth of 3-ply

Further development

How to run them

• Download Toledo Atomchess package (19.2K)
• Latest source code of Toledo Atomchess is in Github

qemu-system-x86_64 -hda toledo_atomchess_disk.bin

Toledo Atomchess source code

 ; ; Toledo Atomchess ; ; by Óscar Toledo Gutiérrez ; ; © Copyright 2015 Óscar Toledo Gutiérrez ; ; Creation: Jan/28/2015 21:00 local time. ; Revision: Jan/29/2015 18:17 local time. Finished. ; Revision: Jan/30/2015 13:34 local time. Debugging finished. ; Revision. Jun/01/2015 10:08 local time. Solved bug where computer bishops never moved over upper diagonals. ; Revision: Oct/06/2015 06:38 local time. Optimized board setup/display, plus tiny bits. ; Revision: Oct/07/2015 14:47 local time. More optimization and debugged. ; Revision: Oct/10/2015 08:21 local time. More optimization. ; Features: ; * Computer plays legal basic chess movements ;) ; * Enter moves as algebraic form (D2D4) (note your moves aren't validated) ; * Search depth of 3-ply ; * No promotion of pawns. ; * No castling ; * No en passant. ; * 446 bytes size (fits in a boot sector) ; Note: I'm lazy enough to write my own assembler instead of ; searching for one, so you will have to excuse my syntax ;) code16 ; Change to org &0100 for COM file org &7c00 ; Housekeeping mov sp,stack cld push cs push cs push cs pop ds pop es pop ss ; Create board mov di,board-8 mov cx,&0108 sr1: push di pop ax and al,&88 ; 0x88 board jz sr2 mov al,&07 ; Frontier sr2: stosb loop sr1 ; Setup board mov si,initial mov di,board mov cl,&08 sr3: lodsb ; Load piece stosb ; Black pieces or al,8 mov [di+&6f],al ; White pieces mov byte [di+&0f],&01 ; Black pawn mov byte [di+&5f],&09 ; White pawn loop sr3 ; ; Main loop ; sr21: call display_board call key2 push di call key2 pop si call sr28 call display_board mov ch,&08 ; Current turn (0=White, 8=Black) call play jmp short sr21 ; ; Computer plays :) ; play: mov bp,-32768 ; Current score push cx ; Current side push bp ; Origin square push bp ; Target square xor ch,8 ; Change side mov si,board sr7: lodsb ; Read square xor al,ch ; XOR with current playing side dec ax cmp al,6 ; Ignore if frontier or empty jnc sr6 or al,al ; Is it pawn? jnz sr8 or ch,ch ; Is it playing black? jnz sr25 ; No, jump sr8: inc ax sr25: dec si add al,&04 mov dl,al ; Total movements of piece in dl and dl,&0c mov bx,offsets-4 xlat add al,displacement&255 mov dh,al ; Movements offset in dh sr12: mov di,si ; Restart target square mov bl,dh mov cl,[bx] sr9: add di,cx and di,&ff or di,board mov al,[si] ; Content of: origin in al, target in ah mov ah,[di] or ah,ah ; Empty square? jz sr10 xor ah,ch sub ah,&09 ; Valid capture? cmp ah,&06 mov ah,[di] jnc sr18 ; No, avoid cmp dh,16+displacement&255 ; Pawn? jc sr19 test cl,1 ; Straight? je sr17 ; Yes, avoid and cancels any double square movement jmp short sr19 sr10: cmp dh,16+displacement&255 ; Pawn? jc sr19 test cl,1 ; Diagonal? jne sr18 ; Yes, avoid sr19: push ax ; Save for restoring in near future mov bl,scores&255 mov al,ah and al,7 cmp al,6 ; King eaten? jne sr20 cmp sp,stack-(5+8+5)*2 ; If in first response... mov bp,20000 ; ...maximum score (probably checkmate/slatemate) je sr26 mov bp,7811 ; Maximum score sr26: add sp,6 ; Ignore values pop cx ; Restore side ret sr20: xlat cbw ; cmp sp,stack-(5+8+5+8+5+8+5+8+5)*2 ; 4-ply depth cmp sp,stack-(5+8+5+8+5+8+5)*2 ; 3-ply depth ; cmp sp,stack-(5+8+5+8+5)*2 ; 2-ply depth ; cmp sp,stack-(5+8+5)*2 ; 1-ply depth jbe sr22 pusha call sr28 ; Do move call play mov bx,sp sub [bx+14],bp ; Substract BP from AX popa sr22: cmp bp,ax ; Better score? jg sr23 ; No, jump xchg ax,bp ; New best score jne sr23 ; Same score? in al,(&40) cmp al,&aa ; Randomize it sr23: pop ax ; Restore board mov [si],al mov [di],ah jg sr18 add sp,4 push si ; Save movement push di sr18: dec ax and al,&07 ; Was it pawn? jz sr11 ; Yes, check special cmp al,&04 ; Knight or king? jnc sr14 ; End sequence, choose next movement or ah,ah ; To empty square? jz sr9 ; Yes, follow line of squares sr16: jmp short sr14 sr11: and cl,&1f ; Advanced it first square? cmp cl,&10 jnz sr14 mov ax,si ; Already checked for move to empty square sub al,&20 cmp al,&40 ; At top or bottom firstmost row? jb sr17 ; No, cancel double-square movement sr14: inc dh dec dl jnz sr12 sr17: inc si sr6: cmp si,board+120 jne sr7 pop di pop si pop cx cmp sp,stack-2 jne sr24 cmp bp,-16384 ; Illegal move? (always in check) jl sr24 ; Yes, doesn't move sr28: movsb ; Do move mov byte [si-1],0 ; Clear origin square sr24: ret ; Display board display_board: mov si,board-8 mov cx,73 ; 1 frontier + 8 rows * (8 cols + 1 frontier) sr4: lodsb mov bx,chars xlat cmp al,&0d ; Is it RC? jnz sr5 ; No, jump add si,7 ; Jump 7 frontier bytes call display ; Display RC mov al,&0a ; Now display LF sr5: call display loop sr4 ret ; Read algebraic coordinate key2: call key ; Read letter add ax,board+127 ; Calculate board column push ax call key ; Read digit pop di shl al,4 ; Substract digit row multiplied by 16 sub di,ax ret key: mov ah,0 ; Read keyboard int &16 ; Call BIOS display: pusha mov ah,&0e ; Console output mov bh,&00 int &10 ; Call BIOS popa and ax,&0f ret initial: db 2,5,3,4,6,3,5,2 scores: db 0,10,50,30,90,30 chars: db ".prbqnk",&0d,".PRBQNK" offsets: db 16,20,8,12,8,0,8 displacement: db -33,-31,-18,-14,14,18,31,33 db -16,16,-1,1 db 15,17,-15,-17 db -15,-17,-16,-32 db 15,17,16,32 ; 64 bytes to say something db "Toledo Atomchess 10oct2015" db " (c)2015 Oscar Toledo G. " db "nanochess.org" ; ; This marker is required for BIOS to boot floppy disk ; ds &7dfe-* ; Change to &02fe for COM file db &55,&aa board: ds 256 ds 256 stack: end 

Toledo Atomchess Reloaded source code

 ; ; Toledo Atomchess reloaded ; ; by Óscar Toledo Gutiérrez ; ; © Copyright 2015 Óscar Toledo Gutiérrez ; ; Creation: 28-ene-2015 21:00 local time. ; Revision: 29-ene-2015 18:17 local time. Finished. ; Revision: 30-ene-2015 13:34 local time. Debugging finished. ; Revision: 26-may-2015. Checks for illegal moves. Handles promotion ; to queen, en passant and castling. ; Revision: 04-jun-2015. At last fully debugged. ; Features: ; * Full chess movements (except promotion only to queen) ; * Enter moves as algebraic form (D2D4) (your moves are validated) ; * Search depth of 3-ply ; * 831 bytes size (fits in two boot sectors) ; Note: I'm lazy enough to write my own assembler instead of ; searching for one, so you will have to excuse my syntax ;) code16 ; Search for "REPLACE" to find changes for COM file org &7c00 ; REPLACE with ORG &0100 ; Housekeeping mov sp,stack cld push cs push cs push cs pop ds pop es pop ss ; Load second sector sr0: push ds push es mov ax,&0201 mov bx,&7e00 mov cx,&0002 xor dx,dx int &13 ; REPLACE with NOP NOP pop es pop ds jb sr0 ; Create board mov bx,board sr1: mov al,bl and al,&88 ; 0x88 board jz sr2 mov al,&07 ; Frontier sr2: mov [bx],al inc bl jnz sr1 ; Setup board mov si,initial mov [enp],si ; Reset en passant state sr3: lodsb ; Load piece mov [bx],al ; Black pieces or al,8 mov [bx+&70],al ; White pieces mov al,&11 mov [bx+&10],al ; Black pawn mov al,&19 mov [bx+&60],al ; White pawn inc bx cmp bl,&08 jnz sr3 ; ; Main loop ; sr21: call display_board call key2 push di call key2 pop si mov ch,&00 ; Current turn (0=White, 8=Black) call play_validate test ch,ch ; Changed turn? je sr21 ; No, wasn't valid call display_board mov ch,&08 ; Current turn (0=White, 8=Black) dec byte [legal] mov word [depth],stack-(4+8+4+8+4+8+4)*2 call play call play_validate jmp short sr21 ; ; Computer plays :) ; play_validate: mov byte [legal],1 mov word [depth],stack-(4+8+4)*2 play: mov bp,-32768 ; Current score push si ; Origin square push di ; Target square xor ch,8 ; Change side mov si,board sr7: lodsb ; Read square xor al,ch ; XOR with current playing side and al,&0f ; Remove moved bit dec ax ; Translate to 0-5 cmp al,6 ; Is it frontier or empty square? jnc sr6 ; Yes, jump or al,al ; Is it pawn? jnz sr8 ; No, jump or ch,ch ; Is it playing black? jnz sr25 ; No, jump sr8: inc ax ; Inverse direction for pawn sr25: dec si mov bx,offsets push ax xlat mov dh,al ; Movements offset in dh pop ax add al,total-offsets xlat mov dl,al ; Total movements of piece in dl sr12: mov di,si ; Restart target square mov bx,displacement mov al,dh xlat mov cl,al ; Current displacement offset in cl sr9: add di,cx and di,&ff or di,board mov al,[si] ; Content of origin square in al mov ah,[di] ; Content of target square in ah and ah,&0f ; Empty square? jz sr10 ; Yes, jump xor ah,ch sub ah,&09 ; Is it a valid capture? cmp ah,&06 mov ah,[di] jnc sr18 ; No, jump to avoid cmp dh,16 ; Moving pawn? jc sr19 test cl,1 ; Straight advance? je sr18 ; Yes, avoid jmp short sr19 sr10: cmp dh,16 ; Moving pawn? jc sr19 ; No, jump test cl,1 ; Diagonal? je sr19 ; No, jump mov bx,si dec bx test cl,2 ; Going left? jne sr29 inc bx inc bx sr29: cmp bx,[enp] ; Is it a valid en passant? jne sr18 ; No, avoid sr19: push ax ; Save origin and target square in stack mov al,ah and al,7 cmp al,6 ; King eaten? jne sr20 cmp sp,stack-(4+8+4)*2 ; If in first response... mov bp,20000 ; ...maximum score (probably checkmate/slatemate) je sr26 mov bp,7811 ; Maximum score sr26: add sp,6 ; Ignore values jmp sr24 sr20: mov bx,scores xlat cbw ; ax = score for capture (guarantees ah = 0) mov bx,[enp] ; bx = current pawn available for en passant cmp sp,[depth] jbe sr22 pusha mov [enp],ax ; En passant not possible mov al,[si] ; Read origin square and al,&0f ; Clear bit 4 (marks piece moved) cmp al,&0e ; Is it a king? je sr36 cmp al,&06 jne sr37 ; No, jump sr36: mov bx,si sub bx,di mov bh,ch ; Create moved rook xor bh,&02 ; cmp bl,2 ; Is it castling to left? jne sr38 ; No, jump mov [di+1],bh ; Put it along king mov [di-2],ah jmp sr37 sr38: cmp bl,-2 ; Is it castling to right? jne sr37 mov [di-1],bh ; Put it along king mov [di+1],ah sr37: cmp al,&09 ; We have a pawn? je sr31 cmp al,&01 jne sr30 ; No, jump sr31: mov bp,sp mov bx,di cmp bl,&10 ; Going to uppermost row? jc sr32 ; Yes, jump cmp bl,&70 ; Going to lowermost row? jc sr33 ; No, jump sr32: xor al,&05 ; Promote to queen add word [bp+14],90 ; Add points for queen sr33: sub bx,si call en_passant_test jnc sr41 mov [bx],ah ; Clean en passant square add word [bp+14],10 ; Add points for pawn jmp sr30 sr41: and bx,&001f ; Moving two squares ahead? jne sr30 ; No, jump mov [enp],di ; Take note of en passant sr30: mov [di],al mov [si],ah ; Clear origin square call play mov bx,sp sub [bx+14],bp ; Substract BP from AX popa ; ; If reached maximum depth then the code can ; come here >without< moving piece ; sr22: mov [temp],ax cmp sp,stack-4*2 ; First ply? jnz sr28 ; No, jump test byte [legal],255 ; Checking for legal move? jz sr28 ; No, jump mov bp,sp cmp si,[bp+4] ; Origin is same? jnz sr23 cmp di,[bp+2] ; Target is same? jnz sr23 cmp ax,-16384 ; Illegal movement? jl sr23 add sp,6 ret ; ; Note: TEST instruction clears carry flag ; en_passant_test: test bl,1 ; Diagonal? je sr42 ; No, jump test byte [di],255 ; Capture? jne sr42 ; Yes, jump test bl,2 ; Going left? stc ; Set carry lea bx,[si-1] jne sr42 lea bx,[si+1] sr42: ret displacement: db -33,-31,-18,-14,14,18,31,33 db -16,16,-1,1 db 15,17,-15,-17 db -15,-17,-16,-32 db 15,17,16,32 scores: db 0,10,50,30,90,30 ; ; This marker is required for BIOS to boot floppy disk ; ds &7dfe-* ; REPLACE with nothing for COM file db &55,&aa ; REPLACE with nothing for COM file ; Start of second sector sr28: cmp bp,ax ; Better score? jg sr23 ; No, jump mov bp,ax ; New best score jne sr27 in al,(&40) cmp al,&55 ; Randomize it jb sr23 sr27: pop ax add sp,4 push si ; Save movement push di push ax sr23: mov [enp],bx ; Restore en passant state pop ax ; Restore board mov [si],al mov [di],ah mov bx,di sub bx,si and al,&07 ; Separate piece cmp al,&01 ; Is it a pawn? jne sr43 call en_passant_test jnc sr43 mov byte [bx],ch ; Clean xor byte [bx],9 ; Restore opponent pawn sr43: cmp al,&06 ; Is it a king? jne sr18 mov bh,ch ; Create unmoved rook xor bh,&12 ; cmp bl,-2 ; Castling to left? jne sr40 mov [di-2],bh mov [di+1],ah jmp sr18 sr40: cmp bl,2 ; Castling to right? jne sr18 mov [di+1],bh mov [di-1],ah sr18: dec ax and al,&07 ; Was it pawn? jz sr11 ; Yes, check special cmp al,&05 ; King? jne sr34 ; No, jump test byte [si],&10 ; King already moved? je sr34 ; Yes, jump cmp word [temp],-4096 ; In check? jl sr34 ; Yes, jump or ah,ah ; Moved to empty square? jne sr34 ; No, jump cmp bl,-1 ; Going left by one? je sr44 cmp bl,1 ; Going right by one? jne sr34 mov bh,[si+3] ; Read rook mov bl,[si+2] ; Read destination square jmp sr46 sr44: test byte [si-3],255 ; Is empty square just right of rook? jne sr34 ; No, jump mov bh,[si-4] ; Read rook mov bl,[si-2] ; Read destination square sr46: test bl,bl ; Ending in empty square? jne sr34 ; No, jump and bh,&17 cmp bh,&12 ; Unmoved rook? je sr9 ; Yes, can move two squares for castling sr34: cmp al,&04 ; Knight or king? jnc sr14 ; End sequence, choose next movement or ah,ah ; To empty square? jz sr9 ; Yes, follow line of squares sr16: jmp short sr14 sr11: and cl,&1f ; Advanced it first square? cmp cl,&10 jnz sr14 sr15: or ah,ah ; Pawn to empty square? jnz sr17 ; No, cancel double-square movement mov ax,si sub al,&20 ; At first top row? cmp al,&40 ; At first bottom row? jb sr17 ; No, cancel double-square movement sr14: inc dh dec dl jnz sr12 sr17: inc si sr6: cmp si,board+120 jne sr7 pop di pop si sr24: xor ch,8 ret display_board: ; Display board call display3 mov si,board sr4: lodsb and al,&0f mov bx,chars xlat call display2 sr5: cmp si,board+128 jnz sr4 ret key2: mov di,board+127 call key add di,ax call key shl al,4 sub di,ax ret key: push di mov ah,0 int &16 push ax call display pop ax and ax,&0f pop di ret display2: cmp al,&0d jnz display display3: add si,7 mov al,&0a call display mov al,&0d display: push si mov ah,&0e mov bh,&00 int &10 pop si ret chars: db ".prbqnk",&0d,".PRBQNK" initial: db &12,&15,&13,&14,&16,&13,&15,&12 offsets: db 16,20,8,12,8,0,8 total: db 4, 4,4, 4,8,8,8 ; Bytes to say something db "Toledo Atomchess reloaded" db "nanochess.org" ds &8000-* ; REPLACE with &0500 for COM file board: ds 256 depth: ds 2 ; Depth for search enp: ds 2 ; En passant square temp: ds 2 ; Working score legal: ds 1 ; Flag indicating legal movement validation ds 249 stack: end

Related links

• Mention of Toledo Atomchess in linux.cn (Chinese).
• Toledo Atomchess has a nice mention in the Retrogamer magazine issue 141.
• I was interviewed because of Toledo Atomchess for the article "The bitter rivalry behind the world's smallest chess program" by Chris Stokel-Walker.
• Toledo Atomchess 6502 crunched in 1K ROM with graphical chessboard for Atari VCS/2600.