Timer and Divider Registers
NOTE
The Timer described below is the built-in timer in the Game Boy. It has nothing to do with the MBC3s battery buffered Real Time Clock - that’s a completely different thing, described in Memory Bank Controllers.
FF04 — DIV: Divider register
This register is incremented at a rate of 16384Hz (~16779Hz on SGB).
Writing any value to this register resets it to $00.
Additionally, this register is reset when executing the stop instruction, and
only begins ticking again once stop mode ends. This also occurs during a
speed switch.
(TODO: how is it affected by the wait after a speed switch?)
Note: The divider is affected by CGB double speed mode, and will increment at 32768Hz in double speed.
FF05 — TIMA: Timer counter
This timer is incremented at the clock frequency specified by the TAC register ($FF07). When the value overflows (exceeds $FF) it is reset to the value specified in TMA (FF06) and an interrupt is requested, as described below.
FF06 — TMA: Timer modulo
When TIMA overflows, it is reset to the value in this register and an interrupt is requested. Example of use: if TMA is set to $FF, an interrupt is requested at the clock frequency selected in TAC (because every increment is an overflow). However, if TMA is set to $FE, an interrupt is only requested every two increments, which effectively divides the selected clock by two. Setting TMA to $FD would divide the clock by three, and so on.
If a TMA write is executed on the same M-cycle as the content of TMA is transferred to TIMA due to a timer overflow, the old value is transferred to TIMA.
FF07 — TAC: Timer control
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
|---|---|---|---|---|---|---|---|---|
| TAC | Enable | Clock select | ||||||
-
Enable: Controls whether
TIMAis incremented. Note thatDIVis always counting, regardless of this bit. -
Clock select: Controls the frequency at which
TIMAis incremented, as follows:Clock select Increment every Frequency (Hz) DMG, SGB2, CGB in single-speed mode SGB1 CGB in double-speed mode 00 256 M-cycles 4096 ~4194 8192 01 4 M-cycles 262144 ~268400 524288 10 16 M-cycles 65536 ~67110 131072 11 64 M-cycles 16384 ~16780 32768
Note that writing to this register may increase TIMA once!