Add EMU_SLOW_FACTOR environment variable to enable slowing down emulator

README.md

@@ -89,6 +89,10 @@ The available environment variables are:
                        properly, some other will ignore this and setup the text
                        mode again.
 
+- `EMU_SLOW_FACTOR`    Set a slowing factor, this forces the CPU to take at
+                       least for the given amount of nanoseconds for every
+                       opcode being executed.
+
 
 Simple Example
 --------------

src/cpu.c

@@ -1,11 +1,14 @@
 #include <stdio.h>
 #include <stdlib.h>
+#include <time.h>
+#include <unistd.h>
 
 #include "cpu.h"
 #include "dbg.h"
 #include "dis.h"
 #include "emu.h"
 #include "os.h"
+#include "env.h"
 
 // Forward declarations
 static void do_instruction(uint8_t code);
@@ -2550,12 +2553,34 @@ static void do_instruction(uint8_t code)
     };
 }
 
+static long slow_ns = 0;
+
+static void slow_cpu(struct timespec *start)
+{
+    if(!slow_ns)
+        return;
+    struct timespec now;
+    clock_gettime(CLOCK_MONOTONIC, &now);
+    long left = slow_ns -
+                (now.tv_sec - start->tv_sec) * 1000000000L -
+                (now.tv_nsec - start->tv_nsec);
+    if (left > 0)
+        clock_nanosleep(CLOCK_MONOTONIC, 0,
+                        &(struct timespec){left / 1000000000L,
+                                           left % 1000000000L}, NULL);
+}
+
 void execute(void)
 {
+    if (getenv(ENV_SLOW))
+        slow_ns = atol(getenv(ENV_SLOW));
     for(; !exit_cpu;)
     {
+        struct timespec start;
+        clock_gettime(CLOCK_MONOTONIC, &start);
         handle_irq();
         next_instruction();
+        slow_cpu(&start);
     }
 }
 

src/env.h

@@ -12,3 +12,4 @@
 #define ENV_LOWMEM    "EMU2_LOWMEM"
 #define ENV_ROWS      "EMU2_ROWS"
 #define ENV_DOSVER    "EMU2_DOSVER"
+#define ENV_SLOW      "EMU_SLOW_FACTOR"