Re: Cycles & Speed Questions

[mjmahon@aol.com (Michael J. Mahon)]

Bryan Parkoff asked:

>Cycles & Speed Questions
>
>    I have run several emulator programs.  I have noticed that both 6502 and
>65816 are running at faster speed than approximately 60MHz.  If they go
>through display, it will only show approximately 30MHz.  I understand that
>emulator can only manipulate the speed through the display, but not through
>6502/65816.
>    For example, running at 60MHz has to do 60 frames per second through
>display that will reduce at 30MHz.  If I force to run at 1MHz, there will be
>some pauses between previous time and present time inside display, but not
>through 6502/65816 because 6502/65816 routines can only handle to use CALL
>and RET functions.
>    Another example, LDA and STA instructions load and store data through
>loop while appear in the screenthat can run at 30MHz because it has to
>update display's frames per second.  If I put JMP instruction toward the
>same address such as 2000: JMP $2000.  It causes to execute endless without
>the display's update that will increase to 60MHz
>    Please explain everything that I can understand how the speed handles
>cycles and time.  What do VBL stand for?  Please advise me if I am wrong.

Various emulators deal with regulating the speed of emulation in different
ways.  The video update rate is not necessarily linked to emulation speed,
although some emulators may delay at each video update to reduce the
effective emulation speed.  This would usually be fine if the objective of
speed regulation is to make games playable.

Other emulators keep a much finer-grain record of emulated cycles and
delay much more frequently to produce a smoother regulated emulation
speed.  This is more a matter of the emulator designer's devotion to
emulation fidelity than any intrinsic characteristic of emulation.

VBL stands for Vertical BLanking, and is a signal which occurs about 60
times per second, coincident with vertical retrace in the video generator.

-michael

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