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/BYTEDEMO.C            | 148 +++++++++++++++++++++
 1 file changed, 148 insertions(+)
 create mode 100644 Bachelor/Mikroprozessorsysteme2/ARM202U/EXAMPLES/EXPLASM/BYTEDEMO.C

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

diff --git a/Bachelor/Mikroprozessorsysteme2/ARM202U/EXAMPLES/EXPLASM/BYTEDEMO.C b/Bachelor/Mikroprozessorsysteme2/ARM202U/EXAMPLES/EXPLASM/BYTEDEMO.C
new file mode 100644
index 0000000..209b4f0
--- /dev/null
+++ b/Bachelor/Mikroprozessorsysteme2/ARM202U/EXAMPLES/EXPLASM/BYTEDEMO.C
@@ -0,0 +1,148 @@
+/* Explanation of byte-reversing in ARM assembler
+
+   for PC only (uses cursor control codes)
+*/
+
+#include <stdio.h>
+
+static void cls()
+{
+  printf("\033[2J");
+}
+
+static void up(int n)
+{
+  while (n > 0)
+  { printf("\033[1A");  --n;
+  }
+}
+
+static void pause()
+{
+  fprintf(stderr, "Please press <Return> to proceeed ");
+  while( fgetc(stdin) != '\n' );
+}
+
+static void prologue()
+{
+    printf("\
+        AMAZING FACTS ABOUT THE ARM - Reversing the bytes in a word\n\
+\n\
+This function reverses the bytes in a word. The method was discovered in\n\
+1986 following a competition between ARM programmers; it requires just 4\n\
+instructions and 1 work register. A method using only 3 instructions per\n\
+word reversed was also found, but it has some set-up overhead and uses a\n\
+2nd register. Can you re-discover this method?\n\
+\n\
+Later, the C compiler was 'taught' to generate exactly the instructions\n\
+needed, from C source. Check this claim using armcc -S -DREV byterev.c\n\
+and examining the assembly code file produced.\n\
+\n\
+unsigned long reverse(unsigned long v)\n\
+{   unsigned long t;\n\
+    t = v ^ ((v << 16) | (v >> 16));   /*  EOR r1,r0,r0,ROR #16  */\n\
+    t &= ~0xff0000;                    /*  BIC r1,r1,#&ff0000    */\n\
+    v = (v << 24) | (v >> 8);          /*  MOV r0,r0,ROR #8      */\n\
+    return v ^ (t >> 8);               /*  EOR r0,r0,r1,LSR #8   */\n\
+}\n\
+\n\
+To see the method in action, press <Return>. Each time you press <Return>\n\
+one step of the reversal process will be executed. The values displayed\n\
+are symbolic, starting with the input word D C B A.\n\
+\n");
+}
+
+static void prelude()
+{
+    printf("\
+        AMAZING FACTS ABOUT THE ARM - Reversing the bytes in a word\n\
+\n\
+unsigned long reverse(unsigned long v)\n\
+{   unsigned long t;\n\
+    t = v ^ ((v << 16) | (v >> 16));   /*  EOR r1,r0,r0,ROR #16  */\n\
+    t &= ~0xff0000;                    /*  BIC r1,r1,#&ff0000    */\n\
+    v = (v << 24) | (v >> 8);          /*  MOV r0,r0,ROR #8      */\n\
+    return v ^ (t >> 8);               /*  EOR r0,r0,r1,LSR #8   */\n\
+}\n\
+\n");
+}
+
+static void show_state_1()
+{
+    printf("\
+        v / r0                 t / r1         original input in v/r0\n\
+\n\
+  +---+---+---+---+      +---+---+---+---+\n\
+  | D | C | B | A |      | x | x | x | x |\n\
+  +---+---+---+---+      +---+---+---+---+\n\
+\n\n\n\n\n\n\n");
+}
+
+static void show_state_2()
+{
+    printf("\
+        v / r0                 t / r1         state after executing \n\
+\n\
+  +---+---+---+---+      +---+---+---+---+    t = v ^ ((v<<16) | (v>>16));\n\
+  | D | C | B | A |      |D^B|C^A|B^D|A^C|\n\
+  +---+---+---+---+      +---+---+---+---+    EOR r1,r0,r0,ROR #16\n\
+\n\n\n\n\n\n\n");
+}
+
+static void show_state_3()
+{
+    printf("\
+  +---+---+---+---+      +---+---+---+---+    t &= ~0xff0000;             \n\
+  | D | C | B | A |      |D^B| 0 |B^D|A^C|\n\
+  +---+---+---+---+      +---+---+---+---+    BIC r1,r1,#&ff0000  \n\
+\n\n\n\n\n\n\n");
+}
+
+static void show_state_4()
+{
+    printf("\
+  +---+---+---+---+      +---+---+---+---+    v = (v << 24) | (v >> 8);\n\
+  | A | D | C | B |      |D^B| 0 |B^D|A^C|\n\
+  +---+---+---+---+      +---+---+---+---+    MOV r0,r0,ROR #8  \n\
+\n\n\n\n\n\n\n");
+}
+
+static void show_state_5()
+{
+    printf("\
+  +---+---+---+---+      +---+---+---+---+    v = v ^ (t >> 8);        \n\
+\n\
+  +---+-^-+-^-+-^-+---+  +---+---+---+---+    EOR r0,r0,r1,LSR #8\n\
+      |D^B| 0 |B^D|A^C|\n\
+      +---+---+---+---+\n\
+\n\
+  +---+---+---+---+\n\
+  | A | B | C | D |      (original input was: D C B A)\n\
+  +---+---+---+---+\n\
+\n");
+}
+
+int main()
+{
+    cls();
+    prologue();
+    pause();
+    cls();
+    prelude();
+    show_state_1();
+    pause();
+    up(13);
+    show_state_2();
+    pause();
+    up(11);
+    show_state_3();
+    pause();
+    up(11);
+    show_state_4();
+    pause();
+    up(11);
+    show_state_5();
+    pause();
+    fputc('\n', stdout);
+    return( 0 );
+}
-- 
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