isa: martins sample

diff --git a/2_isa/src/bittwiddling.c b/2_isa/src/bittwiddling.c
new file mode 100644 (file)
index 0000000..65c6349
--- /dev/null
+++ b/2_isa/src/bittwiddling.c
@@ -0,0 +1,30 @@
+
+#define B0 (0x55555555)
+#define B1 (0x33333333)
+#define B2 (0x0F0F0F0F)
+#define B3 (0x001F001F)
+#define B4 (0x0000003F)
+
+int countbits(int x) {
+       x = (x & B0) + ((x >>  1) & B0);
+       x = (x & B1) + ((x >>  2) & B1);
+
+       x = (x + (x >>  4)) & B2;
+       x = (x + (x >>  8)) /*& B3 */
+       x = (x + (x >> 16)) & B4;
+       return x;
+}
+
+int computeparity(int x) 
+       x ^= x >> 16;
+       x ^= x >> 8;
+       x ^= x >> 4;
+       x &= 0xf;
+       return (0x6996 >> x) & 1;
+}
+
+void main(void) {
+       countbits(B0);
+       computeparity(B0);
+       computeparity(B0+1);
+}

diff --git a/2_isa/src/bittwiddling.s b/2_isa/src/bittwiddling.s
new file mode 100644 (file)
index 0000000..6a2adb1
--- /dev/null
+++ b/2_isa/src/bittwiddling.s
@@ -0,0 +1,92 @@
+
+.data
+.align 32
+;#define B0 (0x55555555)
+B0:
+.dw 0x55555555
+;#define B1 (0x33333333)
+B1:
+.dw 0x33333333
+;#define B2 (0x0F0F0F0F)
+B2:
+.dw 0x0F0F0F0F
+;#define B3 (0x001F001F)
+B3:
+.dw 0x001F001F
+;#define B4 (0x0000003F)
+B4:
+.dw 0x0000003F
+
+.text
+
+;int countbits(int x) {
+countbits:
+       ;x == r1
+       
+       ;load address of B0 into r3
+       ldil r3,low(B0);
+       ldih r3,high(B0);
+
+       ;x = (x & B0) + ((x >>  1) & B0);
+       ;r0 = (x >> 1)
+       lrs r0,r1,1;
+       
+       ;load B0 into r2
+       ldx r2, r3;
+
+       ;(x >> 1) & B0  
+       and r0, r0, r2;
+
+       ;x & B0
+       and r1, r1, r2;
+       
+       ;x = (x & B0) + ((x >>  1) & B0);
+       add r0, r0, r1;
+
+       ;load B1 into r2, use displacment for address of B1 (or add?)
+       ldx r2, (r3)(B1-B0);
+
+       ;x = (x & B1) + ((x >>  2) & B1);
+       lrs r1, r0, 2;
+       and r1, r1, r2;
+
+       and r0, r0, r2;
+
+       add r0, r0, r1;
+
+       ;x = (x + (x >>  4)) & B2;
+       ldx r2, (r3)(B2-B0);
+
+       lrs r1, r0, 4;
+       and r1, r1, r2;
+       and r0, r0, r2;
+
+       add r0, r0, r1;
+
+       ;x = (x + (x >>  8)) /*& B3 */
+       lrs r1, r0, 8;
+       add r0, r0, r1;
+
+       ldx r2, (r3)(B4-B0);
+       ;x = (x + (x >> 16)) & B4;
+       lrs r1, r0, 16;
+       add r0, r0, r1;
+       and r0, r0, r2;
+
+       ;return x;
+       ret;
+;}
+
+int computeparity(int x) 
+       x ^= x >> 16;
+       x ^= x >> 8;
+       x ^= x >> 4;
+       x &= 0xf;
+       return (0x6996 >> x) & 1;
+}
+
+void main(void) {
+       countbits(B0);
+       computeparity(B0);
+       computeparity(B0+1);
+}