From 3b2c40e7f742b043803c97fbc11043b1f50b40f4 Mon Sep 17 00:00:00 2001
From: Niklas Halle <niklas@niklashalle.net>
Date: Wed, 6 May 2020 08:43:09 +0200
Subject: clean up + docu

---
 01_exercise/bootloader.c | 112 +++++++++++++++++++++++++----------------------
 1 file changed, 59 insertions(+), 53 deletions(-)

diff --git a/01_exercise/bootloader.c b/01_exercise/bootloader.c
index 0500ce4..fcdc0f4 100644
--- a/01_exercise/bootloader.c
+++ b/01_exercise/bootloader.c
@@ -2,46 +2,43 @@
 asm(".code16gcc;"
     "jmp $0, $main");
 
-/*
- * 0 black
- * 1 blue
- * 2 green
- * 3 cyan
- * 4 red
- * 5 magenta
- * 6 brown
- * 7 light gray
- * 8 dark gray
- * 9 light blue
- * A light green
- * B light cyan
- * C light red
- * D light magenta
- * E yellow
- * F white
- */
+#define BLACK 0x0
+#define BLUE 0x1
+#define GREEN 0x2
+#define CYAN 0x3
+#define RED 0x4
+#define MAGENTA 0x5
+#define BROWN 0x6
+#define L_GRAY 0x7
+#define GRAY 0x8
+#define L_BLUE 0x9
+#define L_GREEN 0xA
+#define L_CYAN 0xB
+#define L_RED 0xC
+#define L_MAGENTA 0xD
+#define YELLOW 0xE
+#define WHITE 0xF
 
-#define BG_COLOR 0x0
-#define FG_COLOR 0x4
-#define TEXT_COLOR (BG_COLOR << 4 | FG_COLOR)
-
-void putc(char c) {
-    asm volatile("mov $0x0E, %%ah;"
-                 "int $0x10;" ::"al"(c));
-}
+#define BG_COLOR BLACK
+#define FG_COLOR MAGENTA
+#define COLOR(FG, BG) (BG << 4 | FG)
+#define TEXT_COLOR COLOR(FG_COLOR, BG_COLOR)
 
 void putc_color(char c, char color) {
-    asm volatile("mov $0x09, %%ah;"    // using Int 10h AH=09 for attributes
-                 "and $0x00FF, %%bx;"  // erase higher bits of bx
-                 "mov $0x0001, %%cx;"  // write the char once
-                 "int $0x10;"          // we need to manually move the cursor
-                 "mov $0x0300, %%eax;" // get cursor position
-                 "int $0x10;"
-                 "add $0x1, %%dl;"     // raise column by one
-                 "mov $0x0200, %%eax;" // set cursor position
-                 "int $0x10;" ::"al"(c),
-                 "bl"(color)
-                 : "cx", "dx");
+    asm volatile(
+        "mov $0x09, %%ah;"   // using Int 10h AH=09 for attributes
+        "and $0x00FF, %%bx;" // erase higher bits of bx
+        "mov $0x0001, %%cx;" // write the char once
+        "int $0x10;"         // we need to manually move the cursor
+        "mov $0x0300, %%ax;" // get cursor position
+        "int $0x10;"
+        "inc %%dl;" // raise column by one (normally we would need to check for end of
+                    // line; respective end of screen here - for simplicity (and binary
+                    // size) sake, leave that out - will cause bugs)
+        "mov $0x0200, %%ax;" // set cursor position
+        "int $0x10;" ::"al"(c),
+        "bl"(color)
+        : "cx", "dx");
 }
 
 void print(char const *const str) {
@@ -51,30 +48,34 @@ void print(char const *const str) {
 }
 
 void new_line() {
-    putc('\n');
-    putc('\r');
+    // using Int10h AH=0E (teletype print char) here, because it does evaluate \n and \r
+    // (and others)
+    asm volatile("mov $0x0E0a, %%ax;" // \n
+                 "int $0x10;"
+                 "mov $0x0E0d, %%ax;" // \r
+                 "int $0x10;" ::
+                     : "ax");
 }
 
 char getc() {
     char ret;
     asm volatile(
         ".no_key:"
-        "xor %%eax, %%eax;"
+        "xor %%ax, %%ax;"
         "mov $0x0100, %%ax;" // check for keystroke:
         "int $0x16;"         // http://www.ctyme.com/intr/rb-1755.htm
         "jz .no_key;"        // wait for a key
-        ".done:"
 
-        "xor %%eax, %%eax;"
+        "xor %%ax, %%ax;"
         "mov $0x1000, %%ax;" // get keystroke: http://www.ctyme.com/intr/rb-1754.htm
         "int $0x16;"
 
-        "mov %%al, %%bl;"  // temp store char in bl
-        "mov $0400, %%ax;" // clear keyboard buffer:
-        "int $0x16;"       // http://www.ctyme.com/intr/rb-1759.htm
+        "mov %%al, %%bl;"    // temp store char in bl
+        "mov $0x0400, %%ax;" // clear keyboard buffer:
+        "int $0x16;"         // http://www.ctyme.com/intr/rb-1759.htm
         : "=b"(ret)
         :
-        : "eax");
+        : "ax");
     return ret;
 }
 
@@ -94,8 +95,9 @@ void main(void) {
                      : "ax");
 
     while (1) {
-        char buf[9];
-        for (unsigned char i = 0; i < 8; i++) {
+        char buf[9]; // 8 chars + \0
+        unsigned char i = 0;
+        for (; i < 8; i++) { // read at most 8 chars
             char c = getc();
             if (c == '\r') {
                 if (i == 0) { // empty entry
@@ -114,12 +116,16 @@ void main(void) {
                     break;
                 }
             }
-            buf[i] = c;
-            buf[i + 1] = 0;
-            putc_color('*', 0x02); // hard coded green on black
+            buf[i] = c;                           // buffer the char
+            buf[i + 1] = 0;                       // set the following to \0
+            putc_color('*', COLOR(GREEN, BLACK)); // hard coded green on black
+        }
+
+        if (i == 8) {
+            while (getc() != '\r')
+                ; // wait for enter press, ignoring other keys
         }
-        while (getc() != '\r')
-            ;
+
         new_line();
         print(buf);
         new_line();
-- 
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