From 74daa7b891486fdd92cce0c67d076e29b93aeb1c Mon Sep 17 00:00:00 2001
From: "Juan J. Martinez" <jjm@usebox.net>
Date: Thu, 1 Jun 2023 23:51:05 +0100
Subject: Add timer to count ticks

---
 src/main.c  | 12 +++++++++++-
 src/timer.c | 28 ++++++++++++++++++++++++++++
 src/timer.h |  9 +++++++++
 3 files changed, 48 insertions(+), 1 deletion(-)
 create mode 100644 src/timer.c
 create mode 100644 src/timer.h

diff --git a/src/main.c b/src/main.c
index 2b01d3f..d83c18b 100644
--- a/src/main.c
+++ b/src/main.c
@@ -2,14 +2,22 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
+#include <crt0.h>
 
 #include <conio.h>
 
-#include "data.h"
+#include "timer.h"
 #include "vga.h"
+#include "data.h"
+
+/* disable paging because our int handlers are written in C */
+int _crt0_startup_flags = _CRT0_FLAG_LOCK_MEMORY;
 
 int main(int argc, char *argv[])
 {
+    timer_init();
+    atexit(timer_free);
+
     /* set VGA 320x200, 256 col */
     set_mode(0x13);
 
@@ -36,5 +44,7 @@ int main(int argc, char *argv[])
     set_mode(3);
     close_framebuffer();
 
+    printf("tick is %li\n", ticks);
+
     return 0;
 }
diff --git a/src/timer.c b/src/timer.c
new file mode 100644
index 0000000..73675dc
--- /dev/null
+++ b/src/timer.c
@@ -0,0 +1,28 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <go32.h>
+#include <dpmi.h>
+
+/* updates 18.2 times per second by default */
+volatile uint32_t ticks = 0;
+
+static _go32_dpmi_seginfo old_handler, new_handler;
+
+static void timer_handler()
+{
+    ticks++;
+}
+
+void timer_init()
+{
+    _go32_dpmi_get_protected_mode_interrupt_vector(0x1c, &old_handler);
+    new_handler.pm_offset = (unsigned long)timer_handler;
+    new_handler.pm_selector = _go32_my_cs();
+    _go32_dpmi_chain_protected_mode_interrupt_vector(0x1c, &new_handler);
+}
+
+void timer_free()
+{
+    if (_go32_dpmi_set_protected_mode_interrupt_vector(0x1c, &old_handler) == -1)
+        fprintf(stderr, "Failed to free the timer :(\n");
+}
diff --git a/src/timer.h b/src/timer.h
new file mode 100644
index 0000000..22ef6d2
--- /dev/null
+++ b/src/timer.h
@@ -0,0 +1,9 @@
+#ifndef _TIMER_H
+#define _TIMER_H
+
+extern volatile uint32_t ticks;
+
+void timer_init();
+void timer_free();
+
+#endif /* _TIMER_H */
-- 
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