Version analysis of DELAY - Elite on the 6502

This code appears in the following versions (click to see it in the source code):

Cassette, Disc (flight), Disc (docked), 6502 Second Processor, Master and Electron

Code variations between these versions are shown below.

       Name: DELAY
       Type: Subroutine
   Category: Utility routines

Code variation 1 of 7A variation in the comments only

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Cassette, Flight, Docked, 6502SP, Master
Electron
    Summary: Wait for a specified time, in 1/50s of a second
    Summary: Wait for a specified time

Code variation 2 of 7A variation in the comments only

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Cassette, Flight, Docked, 6502SP, Master
Electron
 Wait for the number of vertical syncs given in Y, so this effectively waits
 for Y/50 of a second (as the vertical sync occurs 50 times a second).
 Loop round a convoluted loop-within-loop structure to pass the required
 amount of time.


 Arguments:

Code variation 3 of 7A variation in the comments only

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Cassette, Flight, Docked, 6502SP, Master
Electron
   Y                    The number of vertical sync events to wait for
   Y                    The number of delay loops to run

Code variation 4 of 7A variation in the comments only

This variation is blank in the Disc (flight), Disc (docked), 6502 Second Processor and Master versions.

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Cassette
Electron
 Other entry points:

   DEL8                 Wait for 8/50 of a second (0.16 seconds)

   DELAY-5              Wait for 2/50 of a second (0.04 seconds)
 Other entry points:

   DEL8                 Wait for 30 delay loops' worth of time

   DELAY-5              Wait for 1 delay loop's worth of time

Code variation 5 of 7Related to the Electron version

In the BBC versions, delays are implemented by waiting for a specified number of vertical syncs. The Electron's video system is different, so it has its own dedicated delay routine that isn't based around the screen refresh, but instead wastes time using a convoluted loop-within-loop structure.

See below for more variations related to this code.

This variation is blank in the Disc (flight), Disc (docked), 6502 Second Processor and Master versions.

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Cassette
Electron
 LDY #2                 \ Set Y to 2 vertical syncs

 EQUB &2C               \ Skip the next instruction by turning it into
                        \ &2C &A0 &08, or BIT &08A0, which does nothing apart
                        \ from affect the flags

.DEL8

 LDY #8                 \ Set Y to 8 vertical syncs and fall through into DELAY
                        \ to wait for this long
 LDY #1                 \ Set Y to 1 so we run the delay loop once

 EQUB &2C               \ Skip the next instruction by turning it into
                        \ &2C &A0 &1E, or BIT &1EA0, which does nothing apart
                        \ from affect the flags

.DEL8

 LDY #30                \ Set Y to 30 and fall through into DELAY so we run the
                        \ delay loop 30 times

.DELAY

Code variation 6 of 7Related to the Electron version

See variation 5 above for details.

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 JSR WSCAN              \ Call WSCAN to wait for the vertical sync, so the whole
                        \ screen gets drawn

 DEY                    \ Decrement the counter in Y

Code variation 7 of 7A variation in the comments only

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Cassette, Flight, Docked, 6502SP, Master
Electron
 BNE DELAY              \ If Y isn't yet at zero, jump back to DELAY to wait
                        \ for another vertical sync
 BNE DELAY              \ If Y isn't yet at zero, jump back to DELAY to wait
                        \ for another iteration of the delay loop

 RTS                    \ Return from the subroutine