diff options
| author | Sven Eisenhauer <sven@sven-eisenhauer.net> | 2023-11-10 15:11:48 +0100 |
|---|---|---|
| committer | Sven Eisenhauer <sven@sven-eisenhauer.net> | 2023-11-10 15:11:48 +0100 |
| commit | 33613a85afc4b1481367fbe92a17ee59c240250b (patch) | |
| tree | 670b842326116b376b505ec2263878912fca97e2 /Bachelor/Mikroprozessorsysteme2/ARM202U/EXAMPLES/EXPLASM/BYTEDEMO.C | |
| download | Studium-33613a85afc4b1481367fbe92a17ee59c240250b.tar.gz Studium-33613a85afc4b1481367fbe92a17ee59c240250b.tar.bz2 | |
Diffstat (limited to 'Bachelor/Mikroprozessorsysteme2/ARM202U/EXAMPLES/EXPLASM/BYTEDEMO.C')
| -rw-r--r-- | Bachelor/Mikroprozessorsysteme2/ARM202U/EXAMPLES/EXPLASM/BYTEDEMO.C | 148 |
1 files changed, 148 insertions, 0 deletions
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 );
+}
|