CP/M - screen saver / screen lock

Our precious vintage CP/M machines aren't getting any younger. The same goes for CRT displays. If you are a CP/M programmer, you may be in need of a utility program which will either blank the screen or display random content on the screen to prevent the burnout while you are away from the console taking a break from your project. You may also want a password protected screen lock function with it to protect your precious code from prying eyes.

Here I present something I created out of my own need.
In the process I learned a bit of CP/M system programming in C.

Some interesting problems that I needed to solve due to deficiencies in the standard library of Aztec C 1.05 compiler:

- generating random numbers
- non-blocking keyboard reading
- reading time from the system
- direct cursor addressing

long next = 1;
	/*
	* Aztec C 1.05 doesn't have random generator in the library of functions.
	* Here is something I found on the internet.
	* Will return value 0 - 32767.
	*/
	unsigned rand()
	{
	next = next * 1103515245 + 12345;
	return (unsigned)(next / 65536) % 32768;
	}

/*
	* Clear the screen (ADM31).
	*/
	BlankScr()
	{
	putchar(ESC);
	putchar('*');
	}
	/*
	* Cursor positioning (absolute cursor addressing) for ADM31.
	*/
	GotoXY(col, row)
	char col, row;
	{
	putchar(ESC);
	putchar('=');
	putchar(' '+row);
	putchar(' '+col);
	}
	/*
	* Non-blocking character input.
	* Return 0 if no character is waiting.
	* return a character without echoing if one is waiting.
	*/
	int KeyPress()
	{
	return (bdos (0x06, 0xFF));
	}

/*
	* Get date / time (for rand seed initialization).
	* Returns # of seconds (packed BCD), tptr - address of time stamp.
	* The format of time stamp:
	* DW day ;Day 1 is Jan 1-st 1978
	* DB hr ;Packed BCD
	* DB min ;Packed BCD
	*/
	int GetDtTm(tptr)
	int *tptr;
	{
	return (bdos (0x69, tptr));
	}
	/*
	* Get time string in format hh:mm:ss
	*/
	char *GetTimeStr()
	{
	long dtm;
	char buf[10], hr[3], min[3];
	int sec;
	static char ret[10];
	clear(buf, 10, 0);
	clear(ret, 10, 0);
	hr[2] = 0;
	min[2] = 0;
	sec = GetDtTm(&dtm);
	sprintf(buf, "%08x", dtm >> 16);
	strncpy(hr, buf+6, 2);
	strncpy(min, buf+4, 2);
	sprintf(ret, "%s:%s:%02x", hr, min, sec);
	return (ret);
	}

Program is short and well commented.

Enjoy!

MK'6/20/2019