Add instructions to compile C programs.

README.md

@@ -17,4 +17,22 @@ Here you can find some utilities to jump start your computer from scratch!
 :warning: Mess with electronics only if you know what you are doing. Beware: you may harm your computer, or even yourself! Everything you do is at your own risk.
 
 ## How to
-- to be written
+Just some random notes.
+
+Compile a C program:
+```sh
+zcc +z80 -create-app --no-crt -pragma-define:REGISTER_SP=0xC000 -m -o build/main main.c
+```
+Important: this is a linker-less and C-runtime-less compilation, so be very very careful what you do.
+Example minimal starter:
+```c
+int entrypoint(void) {
+    __asm
+    ld sp,0xc000
+    __endasm;
+
+    main();
+    for (;;)
+      ;
+}
+```

main.c

@@ -0,0 +1,43 @@
+#include <stdint.h>
+
+char* const VIDEO_MEMORY = (char* const)0xd000;
+
+static void vprint(int row, int column, const char* str);
+static void out(uint8_t ioaddr, uint8_t value);
+
+int entrypoint() {
+    __asm
+    ld sp,0xc000
+    __endasm;
+
+    vprint(20,10,"hello world");
+    out(0x81, 0x80);
+    vprint(20,15,"aaaaa");
+
+    for (;;)
+        ;
+}
+
+static void out(uint8_t ioaddr, uint8_t value) {
+    __asm
+    ld  ix,$0000
+    add ix,sp
+    ld  b,(ix + 5)
+    ld  c,(ix + 4)
+    out (c),b
+    __endasm;
+}
+
+static void cls(void) {
+    for (int i = 0; i < 2000; ++i) {
+        VIDEO_MEMORY[i] = 0x20;
+    }
+}
+
+static void vprint(int row, int column, const char* str) {
+    char* p = VIDEO_MEMORY + row * 80 + column;
+    while (*str != '\0') {
+        *p++ = *str++;
+    }
+}
+