Hitachi 6309 Description
Modes of Operation
The 6309 has two modes of operation; 6809 Emulation mode in which the chip acts and executes instructions the same as the 6809, and 6309 Native mode which stores an extra two bytes on the stack when an interrupt (IRQ) occurs, and executes instructions in fewer clock cycles.
When in native mode, the W register (2 additional bytes) is stored (PSHS) on the system stack when an interrupt occurs, it is stored on the stack right after the D (general data) register. Since ALL register values are stored on the system stack when an IRQ (NOT FIRQ - See FIRQ modes for more information) occurs, great care should be taken when writing or patching those routines to run in native mode.
Pull <- CC, A, B, E*, F*, DP, Xhi, Xlo, Yhi, Ylo, Uhi, Ulo, PChi, PClo <- Push
* indicates the additional registers stored on the system stack
When in native mode those interrupt routines which modify the return address by modifying the 10th and 11th byte offsets from the stack (STX 10,S or STY 10,S etc.) will have to be changed to modify the 12th and 13th byte offsets from the stack (STX 12,S or STY 12,S etc.). If those routines are not patched to run in native mode they will either get stuck in a continuous loop or will crash the system due to the fact that they are not returning to the correct address. This poses a MAJOR problem for OS-9 Level II since its main interrupt handling routine relies highly on the changing of the return (PC) address on the stack. Disk read/write and formatting routines also rely heavily on changing the return address during an NMI (Non-Maskable Interrupt).
To patch those routines which do modify the return address, the program or routine must be disassembled or modified with a disk sector editing program. Look for instructions such as STX 10,S or STY 10,S that has an RTI (Return from Interrupt) instruction within the next few lines of the routine. The line containing STX 10,S or STY 10,S should be changed to STX 12,S or STY 12,S respectively.
Remember, after those routines are patched, those programs using them will NOT work in emulation mode and will require native mode to be enabled upon startup.
Native Mode and Timing Loops
There is at least one more problem that needs to be addressed. Those are routines which are dependant on timing loops for accuarate operation. Since the 6309 executes instructions faster when in native mode, those routines that use timing loops would be effected. Since this can pose a problem and can create erratic operation, the delay value or routine will need to be changed for the routine to operate correctly.
Those routines are usually serial-printer routines, cassette read/write timimg routines, software clocks and some disk read/write routines.
Modes of the Fast Interrupt Request (FIRQ)
The designers of the 6309 decided that with the additional instructions and native mode of operation, the FIRQ may be used more than it usually is. With this in mind they decided to allow you to make the FIRQ run the same as the IRQ and store (PSHS) all the current values of the registers on the system stack. Normally, the FIRQ only stores the CC (condition code) and the PC (Program Counter/return address) on the stack, so to keep compatability with the 6809, they included it as a selectable feature in the MD (Mode/status) register.
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