That old X,Y coordinates generation using that ugly way was a real sh*t.

Now it is ways better (it simply does `random number` modulo `screen rows`
- or columns, depends).

There also was an error in comparing snakeX,Y with new piece of food's X,Y
Now really fixed (I hope, at least, since the bug is terribly difficult to
reproduce)

Also improved printing routine: now it can print general hexadecimal numbers,
and can locate them everywhere across first line (and beyond also, since it
is not checked, but if you do, you are a moron). Simply put byte to be print
in accumulator, column in Y index, and then jsr.

snake.asm

@@ -320,48 +320,69 @@ overEndCheck:
     jsr calcTileMem
     lda (tileMem),y     ; read content of head memory location
     cmp foodTile
-    bne checkSelfEat    ; if memory does not contain food, then skip to self-eat test...
+    beq foodEaten       ; if memory does contain food, then perform foodEaten actions,
+    jmp checkSelfEat    ; else just loooong jump to test if I ate myself
+foodEaten:
     ldx length          ; else, increment snake length,
     inx
     stx length
 genFood:
-    ldx random          ; increment random value
+    ldx random
     inx
     stx random
+
     txa
-    sta calcTileX       ; use it as tile X-coordinate
+genFoodX:               ; calculate `random` modulo `screenW`
-    lsr                 ; divide it by 8; maximum expected is 32 (256/8)
+    sec
-    lsr
+    sbc screenW
-    lsr
+    cmp screenW
-    cmp #18             ; test if it is still > 18
+    bcs genFoodX
-    bcc foodNoMod       ; value must be < 18,
+    sta calcTileX
-                        ; because it is the highest value v
+
-                        ; for which v * 40 + 256 < 1000
+    txa
-                        ; this ensures that piece if food will be placed
+genFoodY:               ; calculate `random` modulo 22 (22 = screenH - 1)
-                        ; within visible screen
+    sec
-    sec                 ; if value is > 18, then subtract 18; now it
+    sbc #22
-    sbc #18             ; surely is less than 18
+    cmp #22
-foodNoMod:
+    bcs genFoodY
-    cmp #0
+    clc                 ; add 1 because 1st line can not be used
-    bne foodNoLow       ; if it is 0, then set 1 (avoid first line)
+    adc #1
-    lda #1
+    sta calcTileY
-foodNoLow:
+
-    sta calcTileY       ; use this new value as tile Y-coordinate
     ; Now I have X and Y coordinate for food stored in calcTileX, calcTileY
     ; and I must check it is not the location that I am going to overwrite
     ; with the head in draw snake head...
-    cmp snakeY
+    lda calcTileX
-    bne foodOK
-    lda snakeX
     cmp snakeX
+    bne foodOK
+    lda calcTileY
+    cmp snakeY
     beq genFood
 foodOK:
+    ; debug -- print choosen X,Y for food
+    ldy #$18
+    lda calcTileX
+    jsr printByte
+    ldy #$1b
+    lda calcTileY
+    jsr printByte
+
+    ; debug -- print snake head X,Y
+    ldy #$1f
+    lda snakeX
+    jsr printByte
+    ldy #$22
+    lda snakeY
+    jsr printByte
+
+    ldy #0
     jsr calcTileMem     ; calc food address in memory
     lda (tileMem),y     ; check if memory is empty
     cmp #$20            ; is there a space?
     bne genFood         ; if not, must generate another number
     lda foodTile        ; else, just put that fucking piece of food there
     sta (tileMem),y
+
     lda #$d4
     clc
     adc tileMem + 1
@@ -369,7 +390,9 @@ foodOK:
     lda foodColor
     sta (tileMem),y
 
-    jsr printScore      ; print score
+    ldy #$10
+    lda length
+    jsr printByte      ; print score
     jmp checkEndSelfEat
 checkEndFood:
 
@@ -381,6 +404,7 @@ checkSelfEat:
 checkEndSelfEat:
 
     ; Draw snake head
+    ldy #0
     lda snakeX          ; calc char address in video memory, and put snakeTile
     sta calcTileX
     lda snakeY
@@ -423,6 +447,11 @@ irqalways:
     inx
     stx random
 
+    ; debug -- print current random value
+    ldy #$14
+    lda random
+    jsr printByte
+
     ; Go to original system routine
     jmp $ea31
 
@@ -501,17 +530,11 @@ calcTileEnd:            ; now multiplication is ended, so add X
 
     rts
 
-; Print score
+; Print a byte in hexadecimal
-printScore:
+; A input register for byte to print
-    ; Push registers on stack (save)
+; Y input register for printing colum (on first line)
-    pha
+printByte:
-    txa
+    ; Copy parameter also in X
-    pha
-    tya
-    pha
-
-    ; Take length in A and copy also in X
-    lda length
     tax
 
     lsr ; Take most significant nibble
@@ -519,19 +542,12 @@ printScore:
     lsr
     lsr
     jsr printDigit
-    sta $410        ; print msb char at $410
+    sta $400,y      ; print msb char
 
     txa             ; Take least significant nibble (use previous copy)
     and #$0f
     jsr printDigit
-    sta $411        ; print lsb char at $411
+    sta $401,y       ; print lsb char
-
-    ; Pull registers from stack (restore)
-    pla
-    tay
-    pla
-    tax
-    pla
 
     rts