Version analysis of Main game loop (Part 5 of 6) - Elite on the BBC Micro and NES

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: Main game loop (Part 5 of 6)
       Type: Subroutine
   Category: Main loop
    Summary: Cool down lasers, make calls to update the dashboard
  Deep dive: Program flow of the main game loop
             The dashboard indicators


 This is the first half of the minimal game loop, which we iterate when we are
 docked. This section covers the following:

   * Cool down lasers

   * Make calls to update the dashboard


 Other entry points:

   MLOOP                The entry point for the main game loop. This entry point
                        comes after the call to the main flight loop and
                        spawning routines, so it marks the start of the main
                        game loop for when we are docked (as we don't need to
                        call the main flight loop or spawning routines if we
                        aren't in space)


.MLOOP

Code variation 1 of 8Other (e.g. bug fix, optimisation)

The cassette version disables keyboard interrupts at the start of the minimal game loop, though I'm not entirely sure why.

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

Cassette
 LDA #%00000001         \ Set 6522 System VIA interrupt enable register IER
 STA VIA+&4E            \ (SHEILA &4E) bit 1 (i.e. disable the CA2 interrupt,
                        \ which comes from the keyboard)

Code variation 2 of 8Specific to an individual platform

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

Electron
 LDA LASCT              \ Set A to the value of LASCT, the laser pulse count

 SBC #4                 \ Decrement the value of LASCT by 4

 BCS P%+4               \ If we just reduced LASCT below 0, set it to 0
 LDA #0

 STA LASCT              \ Store the decremented value of X in LASCT, so LASCT
                        \ gets reduced by 4, but not into negative territory

 LDX #&FF               \ Set the stack pointer to &01FF, which is the standard
 TXS                    \ location for the 6502 stack, so this instruction
                        \ effectively resets the stack

Code variation 3 of 8Specific to an individual platform

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

Electron
 INX                    \ Set KEYB = 0 to indicate we are not currently reading
 STX KEYB               \ the keyboard using an OS command

 LDX GNTMP              \ If the laser temperature in GNTMP is non-zero,
 BEQ EE20               \ decrement it (i.e. cool it down a bit)
 DEC GNTMP

.EE20

Code variation 4 of 8Specific to an individual platform

This variation is blank in the Cassette, Disc (flight), Disc (docked) and Electron versions.

6502SP, Master
 LDX LASCT              \ Set X to the value of LASCT, the laser pulse count

 BEQ NOLASCT            \ If X = 0 then jump to NOLASCT to skip reducing LASCT,
                        \ as it can't be reduced any further

 DEX                    \ Decrement the value of LASCT in X

 BEQ P%+3               \ If X = 0, skip the next instruction

 DEX                    \ Decrement the value of LASCT in X again

 STX LASCT              \ Store the decremented value of X in LASCT, so LASCT
                        \ gets reduced by 2, but not into negative territory

.NOLASCT

Code variation 5 of 8A variation in the comments only

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

Master
\LDA QQ11               \ These instructions are commented out in the original
\BNE P%+5               \ source

 JSR DIALS              \ Call DIALS to update the dashboard

Code variation 6 of 8Related to the 6502SP version

The 6502SP version supports a printer (holding CTRL when pressing a red function key will send that screen to the printer). As part of this, the code sends line feeds, which is mainly notable for using the longest label name in the entire source code: dontdolinefeedontheprinternow.

This variation is blank in the Cassette, Disc (flight), Disc (docked), Master and Electron versions.

6502SP
 BIT printflag          \ If bit 7 of printflag is clear (printer output is not
                        \ enabled), jump to dontdolinefeedontheprinternow to
                        \ skip the following (and en route, why not take a
                        \ short moment to admire this, the longest label name in
                        \ the original Elite source code - presumably they got
                        \ longer when development moved to a 6502 second
                        \ processor system, with all that extra memory...)

 BPL dontdolinefeedontheprinternow

 LDA #prilf             \ Send a #prilf command to the I/O processor to print a
 JSR OSWRCH             \ blank line on the printer
 JSR OSWRCH

.dontdolinefeedontheprinternow

 STZ printflag          \ Set the printflag to 0 to disable printing

Code variation 7 of 8Minor and very low-impact

Tap on a block to expand it, and tap it again to revert.

Cassette
Docked
Flight, 6502SP
Master
Electron
 LDA QQ11               \ If this is a space view, skip the following four
 BEQ P%+11              \ instructions (i.e. jump to JSR TT17 below)

 AND PATG               \ If PATG = &FF (author names are shown on start-up)
 LSR A                  \ and bit 0 of QQ11 is 1 (the current view is type 1),
 BCS P%+5               \ then skip the following instruction

 JSR DELAY-5            \ Delay for 8 vertical syncs (8/50 = 0.16 seconds), to
                        \ slow the main loop down a bit
 LDA QQ11               \ If this is a space view, skip the following two
 BEQ P%+7               \ instructions (i.e. jump to JSR TT17 below)

 LDY #2                 \ Wait for 2/50 of a second (0.04 seconds), to slow the
 JSR DELAY              \ main loop down a bit
 LDA QQ11               \ If this is a space view, skip the following five
 BEQ P%+13              \ instructions (i.e. jump to JSR TT17 below)

 AND PATG               \ If PATG = &FF (author names are shown on start-up)
 LSR A                  \ and bit 0 of QQ11 is 1 (the current view is type 1),
 BCS P%+7               \ then skip the following two instructions

 LDY #2                 \ Wait for 2/50 of a second (0.04 seconds), to slow the
 JSR DELAY              \ main loop down a bit
 LDA QQ11               \ If this is a space view, jump to plus13 to skip the
 BEQ plus13             \ following five instructions

 AND PATG               \ If PATG = &FF (author names are shown on start-up)
 LSR A                  \ and bit 0 of QQ11 is 1 (the current view is type 1),
 BCS P%+7               \ then skip the following two instructions

 LDY #2                 \ Wait for 2/50 of a second (0.04 seconds), to slow the
 JSR DELAY              \ main loop down a bit

.plus13
 LDA QQ11               \ If this is a space view, skip the following four
 BEQ P%+11              \ instructions (i.e. jump to JSR TT17 below)

 AND PATG               \ If PATG = &FF (author names are shown on start-up)
 LSR A                  \ and bit 0 of QQ11 is 1 (the current view is type 1),
 BCS P%+5               \ then skip the following instruction

 JSR DELAY-5            \ Delay for 1 delay loop, to slow the main loop down a
                        \ bit

Code variation 8 of 8A variation in the comments only

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked, 6502SP, Master
Electron
 JSR TT17               \ Scan the keyboard for the cursor keys or joystick,
                        \ returning the cursor's delta values in X and Y and
                        \ the key pressed in A
 JSR TT17               \ Scan the keyboard for the cursor keys, returning the
                        \ cursor's delta values in X and Y and the key pressed
                        \ in A