more

projects/snow/main.s

@@ -3,6 +3,9 @@
         .text
 
         .equ        NUM_FLAKES, 150
+        .equ        MAX_COLUMN, 80
+        .equ        MAX_LINE, 24
+        .equ        MAX_LINE_X2, 48
 
 storm   .req        x27
 
@@ -34,7 +37,7 @@ main:   stp         x29, x30, [sp, -16]!
                                         // allocated memory will be
                                         // preserved in x27.
         ldr         x2, [sp], 16        // Restore the allocation size.
-        mov         w1, xzr             // Fill with zero.
+        mov         w1, wzr             // Fill with zero.
         bl          memset              // x0 still had base address.
 
 99:     mov         x0, storm
@@ -54,9 +57,47 @@ bye:    ldp         x27, x28, [sp], 16
 */
 ResetFlake:
         stp         x20, x30, [sp, -16]!
+        mov         x20, x0
+
+        // Initialize Column --- 1 <= random value <= 80 
+        bl          rand
+        mov         x1, MAX_COLUMN
+        bl          mod
+        add         w1, w1, 1
+        str         w1, [x20, 4]
+
+        // Initialize Line --- -(1 <= random value <= 48)
+
+        bl          rand
+        mov         x1, MAX_LINE_X2
+        bl          mod
+        add         w1, w1, 1
+        neg         w1, w1
+        str         w1, [x20]
+
         ldp         x20, x30, [sp], 16
         ret
 
+/* mod(a, b) - implements a % b - AARCH64 lacks a mod instruction.
+   A strange place to economize, but there you have it.
+
+   a comes to us in x0
+   b comes to us in x1
+
+   method:
+    * integer divide a by b
+      for example - 5 % 3 would need 5 // 3 yielding 1
+    * multiply result by b
+      1 * 3 is 3
+    * subtract result from a
+      5 - 3 is 2 and that's our return value.
+*/
+
+mod:	sdiv 	x2, x0, x1         // x2 gets a // b
+        msub	x0, x2, x1, x0     // x0 gets a - a // b * b	
+        ret
+
+
             .data
 bad_malloc: .asciz      "Allocation of flakes has failed"