slow the game around without adding bytes (#68)

README.md

@@ -96,10 +96,10 @@ An **empty** C program generated with `gcc -Os -w -xc - <<< "main;"` on linux-x8
   <br/>
 
 ```
-c53000b80000cd108b3f8d22e5402
-1c3300fbbd0077af5e4606bc00ad4
-14d5449801c739dfad10257bd9893
-a74de880f83eb5079f95b88277bd8
+c53800b80000cd108b3789fcba0a2
+0e54021c3000fbbd00778f5e460f6
+ead414d5449801f039d8ab96d01c7
+1d672e029d3880f75fa5b883779db
 ```
 </details>
 

demo/main.js

@@ -16,7 +16,7 @@ xhr.open('GET', 'snake.com', true);
 xhr.responseType = 'arraybuffer';
 
 xhr.onload = _ =>
-    Dos(canvas, { cycles: 2, onprogress: ()=>{} }).ready((fs, main) =>
+    Dos(canvas, { cycles: 6, onprogress: ()=>{} }).ready((fs, main) =>
         fs.extract(URL.createObjectURL(new Blob([zip(new Uint8Array(xhr.response))]))).then(() =>
             main(["main.com"]).then(ci => {
                 swipedetect(swipedir => swipedir && ci.simulateKeyPress(36 + swipedir));

snake.asm

@@ -1,38 +1,41 @@
 ; register usage during main loop
 ; DS: 0xB800, segment of screen buffer
 ; BX: 0x7D0, screen size (40x25x2 bytes), used in food generation, edge checks, also used for screen accesses but constantly reinitialized
-; DI: position of the snake head
-; SI: pointer to memory location where the current position of the snake head is stored (actual pointer is BP+SI because it defaults to SS)
-lds si, [bx+si]         ; SI=0x100 and BX=0x0 at program start in most DOS versions, this initializes DS and SI (machine code at 0x100 is c5 30 00 b8)
+; DX: 0x200A, DH used for clearing old snake tiles, DL used for magic IMUL and DX used for wall building
+; SI: position of the snake head
+; DI: pointer to memory location where the current position of the snake head is stored
+; SP: pointer to memory location where the current position of the snake tail is stored
+lds di, [bx+si]         ; SI=0x100 and BX=0x0 at program start, this initializes DS and DI (machine code at 0x100 is c5 38 00 b8)
 db 0x0                  ; dummy byte for LDS. this with 'mov ax, 0x0' is actually 'add [bx+si+0x0], bh' but player dies immediately and loop returns to start
 start:                  ; reset game
     mov ax, 0x0         ;   set video mode (AH=0x00) to mode 0 (AL=0x0), text mode 40x25 16 colors
     int 0x10            ;     using BIOS interrupt call, also clears the screen
-    mov di, [bx]        ;   reset head position, BX always points to a valid screen position containing 0x720 after setting video mode
-    lea sp, [bp+si]     ;   set stack pointer (tail) to current head pointer
+    mov si, [bx]        ;   reset head position, BX always points to a valid screen position containing 0x720 after setting video mode
+    mov sp, di          ;   set SP to current head pointer
+    mov dx, 0x200A      ; DH set to empty space, DL to 0xA, DX satisfies DX*8%0x10000=0x50 so good for the wall building
 .food:                  ; create new food item
     in ax, 0x40         ;   read 16 bit timer counter into AX for randomization
     and bx, ax          ;     mask with BX to make divisible by 4 and less than or equal to screen size
-    xor [bx], cl        ;   place food item and check if position was empty by applying XOR with CL (assumed to be 0xFF)
+    add [bx], cl        ;   place food item and check if position was empty by applying ADD with CL (assumed to be 0xFF)
 .input:                 ; handle keyboard input
     mov bx, 0x7D0       ; initialize BX
-    jp .food            ;     if position was occupied by snake or wall in food generation => try again, if we came from main loop PF=0
+    js .food            ;     if position was occupied by snake or wall in food generation => try again, if we came from main loop SF=0
     in al, 0x60         ;   read scancode from keyboard controller - bit 7 is set in case key was released
-    imul ax, BYTE 0xA   ;   we want to map scancodes for arrow up (0x48/0xC8), left (0x4B/0xCB), right (0x4D/0xCD), down (0x50/0xD0) to movement offsets
+    imul dl             ;   we want to map scancodes for arrow up (0x48/0xC8), left (0x4B/0xCB), right (0x4D/0xCD), down (0x50/0xD0) to movement offsets
     aam 0x14            ;     IMUL (AH is irrelevant here), AAM and AAD with some magic constants maps up => -80, left => -2, right => 2, down => 80
     aad 0x44            ;     using arithmetic instructions is more compact than checks and conditional jumps
     cbw                 ;     but causes weird snake movements though with other keys
-    add di, ax          ; add offset to head position
-    cmp di, bx          ; check if head crossed vertical edge by comparing against screen size in BX
-    lodsw               ; load 0x2007 into AX from off-screen screen buffer and advance head pointer
-    adc [di], ah        ; ADC head position with 0x20 to set snake character
-    jnp start           ;   if it already had snake or wall in it or if it crossed a vertical edge, PF=0 from ADC => game over
-    mov [bp+si], di     ; store head position, use BP+SI to default to SS
-    jz .food            ; if food was consumed, ZF=1 from ADC => generate new food
+    add ax, si          ; set AX to new head position
+    cmp ax, bx          ; check if head crossed vertical edge by comparing against BX
+    stosw               ; store head position (SS=ES by default) and advance head pointer
+    xchg ax, si         ; save new head position to SI
+    rcr BYTE [si], 0x1  ; RCR head position to set snake character
+    jno start           ;   if it already had snake or wall in it or if it crossed a vertical edge, OF=0 from RCR => game over
+    jc .food            ; if food was consumed, CF=1 from RCR => generate new food
 .wall:                  ; draw an invisible wall on the left side
+    sub bx, dx          ;   go one line and 0x2000 bytes backwards (the added bytes wrap nicely as 0x10000%0x2000=0)
     mov [bx], cl        ;   store wall character
-    sub bx, BYTE 0x50   ;   go one line backwards
-    jns .wall           ; jump to draw the next wall
+    jnz .wall           ; jump to draw the next wall
     pop bx              ; no food was consumed so pop tail position into BX
-    mov [bx], ah        ; clear old tail position on screen
-    jnp .input          ; loop to keyboard input, PF=0 from SUB
+    mov [bx], dh        ; clear old tail position on screen
+    jns .input          ; loop to keyboard input, SF=0 from SUB