RTC High-Resolution Timer

Unit: RTCTimer

Real-Time Clock (IRQ8) timer for delta-time game loops. No conflict with HSC (IRQ0).

Constants

const
  CMOS_PORT = $70;
  CMOS_DATA = $71;
  PIC1_OCW1 = $21;
  PIC2_OCW1 = $A1;

Globals

var
  RTC_Ticks: LongInt;      { Tick counter }
  RTC_Freq_Hz: Word;       { Current frequency }
  OldInt70: Pointer;       { Original IRQ8 handler }

Functions

procedure InitRTC(Freq: Word);               { Freq: 2-8192 Hz (power of 2) }
procedure DoneRTC;
function GetTimeSeconds: Real;               { Current time in seconds }

Example (Delta-Time Game Loop)

uses RTCTimer, VGA, Sprite;

var
  LastTime, CurrentTime, DeltaTime: Real;
  Player: TSpriteInstance;
begin
  InitRTC(1024);                             { 1024 Hz = ~1ms precision }
  InitVGA;

  LastTime := GetTimeSeconds;
  while Running do
  begin
    CurrentTime := GetTimeSeconds;
    DeltaTime := CurrentTime - LastTime;     { Time in seconds }
    LastTime := CurrentTime;

    UpdateSprite(Player, DeltaTime);         { Animate sprite }
    DrawSprite(@Player, GetScreenBuffer);

    { Cap at 60 FPS }
    while GetTimeSeconds - CurrentTime < 0.0166 do;
  end;

  DoneVGA;
  DoneRTC;                                   { CRITICAL: Restore IRQ8 }
end;

Frequency Values

Freq (Hz)	Precision	Use Case
2	500 ms	Low-precision timers
64	~15.6 ms	60 FPS cap (~16ms)
256	~3.9 ms	General game loops
1024	~0.98 ms	High-precision timing
8192	~0.12 ms	Benchmark/profiling

Critical Notes

1. Call DoneRTC: MUST call before exit or system hangs
2. IRQ8 Isolation: No conflict with HSC (IRQ0) - use for delta-time with music
3. IRQ2 Never Mask: IRQ2 is PIC cascade - masking disables all IRQ8-15 (mouse, RTC, etc.)
4. ExitProc: Install handler to call DoneRTC on Ctrl+C

Comparison: RTC vs PIT

Feature	PIT Timer 0 (IRQ0)	RTC (IRQ8)
Frequency	18.2 Hz (fixed)	2-8192 Hz
HSC Conflict	❌ YES	✅ NO
Precision	~55ms	<1ms at 1024 Hz
Use Case	System tick	Game delta-time

Notes

• RTC uses CMOS registers (ports $70/$71)
• IRQ8 on slave PIC (IRQ15-8)
• GetTimeSeconds returns RTC_Ticks / RTC_Freq_Hz
• DeltaTime convention: Use Real type, seconds (not milliseconds)