Game of Life 32b

This writeup is still in the works! Come back later to get information on all the mean tricks ;) Meanwhile you can download and comment the intro.

Original version : 65 bytes

We'll start with the old 65 bytes version and bring it down to 32 bytes.

It will help to understand what the core algorithm does, before optimizing it. I will not go into the details of random number generation and key handling since these parts are removed in the final version anyway. The core routine computes the [https://en.wikipedia.org/wiki/Conway%27s_Game_of_Life#Rules "normal" game of life rules, but with a twist. Instead of regarding only neighbour cells inside a 3x3 neighbourhood , ALL cells are taken into consideration, and the rules are reinterpreted as:

• If the number of cells is 3, the center cell will be alive.
• If the number of cells is 4, the center cell keeps its state.
• Otherwise, the cell dies (or stays dead).

Like in other (trivial) implementations, the 2D space is parsed cell by cell, from left to right, and from top to bottom. Since the game of life does not work "in situ" (updating the current cell instantly will lead to wrong results of following calculations), current cells are "marked", and when the calculations are advanced far enough that the cell in question does not influence any calculation of the current iteration, it will be "corrected" to the target value of the next iteration. The summation is as usual, an inner loop, adding up 3 cells of one column, and the outer loop, shifting from right (+1) to the left (-1), thus adding up 9 cells of a 3x3 neighbourhood.

At the start of the loop, there is already the first "trick" happening. The register of summation is not properly cleaned, but at this point it can either contain 0 or 32 from the instruction and al,0x20. If an arbitrary amount of cells has this on bit set, that won't hurt the calculation because of a special property of the rcl instruction. "The processor restricts the count to a number between 0 and 31 by masking all the bits in the count operand except the 5 leastsignificant bits."

; http://read.pudn.com/downloads208/sourcecode/asm/981812/LIFE65.ASM__.htm
; Life simulator, 72 bytes  - Vladislav Kaipetsky and Tenie Remmel 
;                 65 bytes  - Mark Andreas 
 
; If no args, regs on startup are: 
 
; AX = BX = 0000h 
; SI = IP = 0100h 
; DI = SP = FFFEh 
 
IDEAL 
MODEL TINY 
P386 
CODESEG 
ORG 100h 
 
Start:  int     1ah             ; ah=00: cx=hours, dx=tic counter 
 
        mov     al,13h          ; Set mode 13h 
        int     10h 
 
        xchg    dx,ax 
 
        push    09000h          ; DS = last 64K segment 
        pop     ds 
        push    0A000h          ; ES = video memory 
        pop     es 
                                ; BX is already zero 
RandLoop: 
        rol     ax,1            ; Generate random number 
        adc     [bx],al 
        dec     bx 
        jnz     RandLoop 
 
; BX will not be equal to 3 the first time this loop is executed, but 
; it will be for all other times.   As SI = 0100h and DI = FFFEh on 
; startup, SI - DI will be equal to 258. 
 
LifeLoop: 
        xchg    cx,ax 
AccLoop: 
        add     cl,[di+bx-64]   ; Add in this column 
        add     cl,[si+bx-2] 
        add     cl,[si+bx+318] 
        dec     bx              ; Loop back 
        jnz     AccLoop 
 
        mov     al,[si]         ; Get center cell, set pixel 
        stosb 
        stc                     ; 3 = birth, 4 = stay (tricky): 
        rcr     al,cl           ; 1.00?0000x --> 0.0x100?00 (rcr 3) 
        and     al,20h          ; ^carry   |         ^ 
                                ;          +---> 0.00x100?0 (rcr 4) 
        or      [si-1],al       ; Add in new cell     ^ 
        shr     [byte di-65],5  ; Shift previous value 
 
        mov     bl,3            ; 3 iterations in AccLoop 
        inc     si              ; Loop while not zero 
        jnz     LifeLoop 
 
        mov     ah,1            ; Check for key 
        int     16h 
        jz      LifeLoop        ; Loop if no key 
 
        xchg    ax,bx           ; Set text mode 
        int     10h 
        ret                     ; Return 
End     Start

Remove key handler and RNG : 44 bytes

Start:  
 
        mov     al,93h          ; Set mode 13h 
        int     10h 
 
        push    09000h          ; DS = last 64K segment 
        pop     ds 
        push    0A000h          ; ES = video memory 
        pop     es 
                                ; BX is already zero  
LifeLoop: 
        xchg    cx,ax 
AccLoop: 
        add     cl,[di+bx-64]   ; Add in this column 
        add     cl,[si+bx-2] 
        add     cl,[si+bx+318] 
        dec     bx              ; Loop back 
        jnz     AccLoop 
 
        ;mov     al,[si]         ; Get center cell, set pixel 
		lodsb
        stosb 
        stc                     ; 3 = birth, 4 = stay (tricky): 
        rcr     al,cl           ; 1.00?0000x --> 0.0x100?00 (rcr 3) 
        and     al,20h          ; ^carry   |         ^ 
                                ;          +---> 0.00x100?0 (rcr 4) 
        or      [si-1],al       ; Add in new cell     ^ 
        shr     [byte di-65],5  ; Shift previous value 
 
        mov     bl,3            ; 3 iterations in AccLoop 
        ; inc     si              ; Loop while not zero 
        jmp short     LifeLoop

Switching to Textmode : 39 bytes

		 push 0xb800
		 pop ds
LifeLoop:
         stc                   ; 3 = birth, 4 = stay (tricky):
         rcr al,cl             ; 1.00?0000x --> 0.0x100?00 (rcr 3)
         and al,20h            ; ^carry   |         ^
         or [si-2],al          ; Add in new cell     ^
         shr byte [si-160-6],5    ; Shift previous value
         mov bl,6 
		xchg cx,ax
AccLoop: add  cl,[si+bx-160-4]    ; Add in this column
         add  cl,[si+bx-4]
         add  cl,[si+bx+160-4]
         dec  bx               ; Loop back
         dec  bx               ; Loop back
         jnz  AccLoop
		 lodsw
         jmp LifeLoop