Divide by 20 only with shifting and substract

1 files changed, 60 insertions, 0 deletions

diff --git a/test/div20.c b/test/div20.c
new file mode 100644
index 0000000..1003ab8
--- /dev/null
+++ b/test/div20.c
@@ -0,0 +1,60 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+
+
+/*
+Page about div
+http://bisqwit.iki.fi/story/howto/bitmath/#DivAndModDivisionAndModulo
+*/
+
+uint8_t div20u8( uint16_t num )
+{
+	uint16_t a = num;
+	uint16_t b = 20;
+	uint16_t r = 0;
+	uint16_t m = 1;
+	while ( b < a )
+	{
+		b = b << 1;
+		m = m << 1;
+	}
+	do 
+	{
+		if ( a >= b )
+		{
+			a = a - b;
+			r = r + m;
+		}
+		b = b >> 1;
+		m = m >> 1;
+	} while ( m != 0 );
+	return (uint8_t)r;
+}
+
+int main()
+{
+	uint32_t a = 10000;
+	uint32_t b = 5;
+	uint32_t r = 0;
+	uint32_t m = 1;
+	printf("%04x(%d) %04x(%3d) %04x(%3d) %04x(%3d)\n",a,a,b,b,r,r,m,m);
+	while ( b < a )
+	{
+		b = b << 1;
+		m = m << 1;
+		printf("%04x(%3d) %04x(%3d) %04x(%3d) %04x(%3d)\n",a,a,b,b,r,r,m,m);
+	}
+	do
+	{
+		if ( a >= b )
+		{
+			a = a - b;
+			r = r + m;
+		}
+		b = b >> 1;
+		m = m >> 1;
+		printf("%04x(%3d) %04x(%3d) %04x(%3d) %04x(%3d)\n",a,a,b,b,r,r,m,m);
+	} while ( m != 0 );
+	return 0;
+}
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