From 33613a85afc4b1481367fbe92a17ee59c240250b Mon Sep 17 00:00:00 2001
From: Sven Eisenhauer <sven@sven-eisenhauer.net>
Date: Fri, 10 Nov 2023 15:11:48 +0100
Subject: add new repo

---
 .../ARM202U/EXAMPLES/EXPLASM/DIVC.C                | 213 +++++++++++++++++++++
 1 file changed, 213 insertions(+)
 create mode 100644 Bachelor/Mikroprozessorsysteme2/ARM202U/EXAMPLES/EXPLASM/DIVC.C

(limited to 'Bachelor/Mikroprozessorsysteme2/ARM202U/EXAMPLES/EXPLASM/DIVC.C')

diff --git a/Bachelor/Mikroprozessorsysteme2/ARM202U/EXAMPLES/EXPLASM/DIVC.C b/Bachelor/Mikroprozessorsysteme2/ARM202U/EXAMPLES/EXPLASM/DIVC.C
new file mode 100644
index 0000000..3a5d85c
--- /dev/null
+++ b/Bachelor/Mikroprozessorsysteme2/ARM202U/EXAMPLES/EXPLASM/DIVC.C
@@ -0,0 +1,213 @@
+/*  Optimal divide-by-contstant code generator
+    Advanced RISC Machines
+*/
+
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#define BANNER		"generated by divc 1.02 (Advanced RISC Machines)"
+#define DATE		"27 Jan 94"
+
+#define DIVIDE_BY_2_MINUS_1		0
+#define DIVIDE_BY_2_PLUS_1		1
+
+
+typedef unsigned int uint;
+
+
+uint log2( uint n )
+{
+  uint bit, pow, logn;
+
+  for( bit = 0, pow = 1; bit < 31; bit++, pow <<= 1 )
+  {
+    if( n == pow ) logn = bit;
+  }
+
+  return( logn );
+}
+
+
+int powerof2( int n )
+{
+  return( n == ( n & (-n) ) );
+}
+
+
+void dodiv2m1( uint logd, uint logmsb )
+{
+  /* Output instructions to do division by 2^n - 1 */
+  printf( "\tMOV     a1, a1, lsr #1\n" );
+
+  while( logd < 32 )
+  {
+    printf( "\tADD     a1, a1, a1, lsr #%d\n", logd );
+    logd <<= 1;
+  }
+  printf( "\tMOV     a1, a1, lsr #%d\n", logmsb - 1 );
+}
+
+
+void dodiv2p1( uint logd, uint logmsb )
+{
+  /* Output instructions to do division by 2^n + 1 */
+  printf( "\tSUB     a1, a1, a1, lsr #%d\n", logd );
+
+  while( logd < 16 )
+  {
+    logd <<= 1;
+    printf( "\tADD     a1, a1, a1, lsr #%d\n", logd );
+  }
+
+  printf( "\tMOV     a1, a1, lsr #%d\n", logmsb );
+}
+
+
+void loada4( uint type, uint lsb, uint msb )
+{
+  /* Constant is too big to be used as an immediate constant, */
+  /* so load it into register a4.                             */
+  printf( "\tMOV     a4, #0x%x\n", msb );
+
+  switch( type )
+  {
+  case DIVIDE_BY_2_MINUS_1:
+    printf( "\tSUB     a4, a4, #0x%x\n", lsb );
+    break;
+
+  case DIVIDE_BY_2_PLUS_1:
+    printf( "\tADD     a4, a4, #0x%x\n", lsb );
+    break;
+
+  default:
+    fputs( "Internal error", stderr );
+  }
+}
+
+
+void divideby2( uint type, uint n, uint lsb, uint msb )
+{
+  uint loglsb;
+  uint logmsb;
+  uint usinga4;
+
+  loglsb = log2( lsb );
+  logmsb = log2( msb );
+
+  printf( "; %s [%s]\n\n", BANNER, DATE );
+  printf( "\tAREA |div%d$code|, CODE, READONLY\n\n", n );
+  printf( "\tEXPORT udiv%d\n\n", n );
+  printf( "udiv%d\n", n );
+  printf( "; takes argument in a1\n" );
+  printf( "; returns quotient in a1, remainder in a2\n" );
+  printf( "; cycles could be saved if only divide or remainder is required\n" );
+
+  usinga4 = ( n >> loglsb ) > 255;
+  if( usinga4 )
+  {
+    loada4( type, lsb, msb );
+    printf( "\tSUB     a2, a1, a4\n" );
+  }
+  else
+  {
+    printf( "\tSUB     a2, a1, #%d\n", n );
+  }
+
+  /* 1/n as a binary number consists of a simple repeating pattern     */
+  /* The multiply by 1/n is expanded as a sequence of ARM instructions */
+  /* (there is a rounding error which must be corrected later)         */
+  switch( type )
+  {
+  case DIVIDE_BY_2_MINUS_1:
+    dodiv2m1( logmsb - loglsb, logmsb );
+    /* Now do multiply-by-n */
+    printf( "\tRSB     a3, a1, a1, asl #%d\n", logmsb - loglsb );
+    break;
+
+  case DIVIDE_BY_2_PLUS_1:
+    dodiv2p1( logmsb - loglsb, logmsb );
+    /* Now do multiply-by-n */
+    printf( "\tADD     a3, a1, a1, asl #%d\n", logmsb - loglsb );
+    break;
+
+  default:
+    fputs( "Internal error", stderr );
+  }
+
+  /* Subtract from adjusted original to obtain remainder */
+  printf( "\tSUBS    a2, a2, a3, asl #%d\n", loglsb );
+
+  /* Apply corrections */
+  printf( "\tADDPL   a1, a1, #1\n" );
+  if( usinga4 )
+  {
+    printf( "\tADDMI   a2, a2, a4\n" );
+  }
+  else
+  {
+    printf( "\tADDMI   a2, a2, #%d\n", n );
+  }
+
+  /* Additional test required for divide-by-3, as result could be  */
+  /* off by 2 lsb due to accumulated rounding errors.              */
+  if( n == 3 )
+  {
+    printf( "\tCMP     a2, #3\n" );
+    printf( "\tADDGE   a1, a1, #1\n" );
+    printf( "\tSUBGE   a2, a2, #3\n" );
+  }
+
+  printf( "\tMOV     pc, lr\n\n" );
+  printf( "\tEND\n" );
+}
+
+
+int main( int argc, char *argv[] )
+{
+  if( argc != 2 )
+  {
+    printf( "Usage: divc <n>\n" );
+    printf( "Generates optimal ARM code for divide-by-constant\n" );
+    printf( "where <n> is one of (2^n-2^m) or (2^n+2^m) eg. 10\n" );
+    printf( "Advanced RISC Machines [%s]\n", DATE );
+  }
+  else
+  {
+    int num;
+
+    num = atoi( argv[ 1 ] );
+    if( num <= 1 )
+    {
+      fprintf( stderr, "%d is not sensible\n", num );
+    }
+    else
+    {
+      uint lsb = 1;
+
+      /* find least-significant bit */
+      while( ( num & lsb ) == 0 )
+      {
+        lsb <<= 1;
+      }
+
+      if( powerof2( num ) )
+      {
+        fprintf( stderr, "%d is an easy case\n", num );
+      }
+      else if( powerof2( num + lsb ) )
+      {
+        divideby2( DIVIDE_BY_2_MINUS_1, num, lsb, num + lsb );
+      }
+      else if( powerof2( num - lsb ) )
+      {
+        divideby2( DIVIDE_BY_2_PLUS_1, num, lsb, num - lsb );
+      }
+      else
+      {
+        fprintf( stderr, "%d is not one of (2^n-2^m) or (2^n+2^m)\n", num );
+      }
+    }
+  }
+  return( 0 );
+}
-- 
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