Skip to navigation

Elite on the BBC Micro

Version analysis of Main game loop (Part 5 of 6)

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

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 7Other (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 7Specific 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 7Specific 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 7Specific 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
 JSR DIALS              \ Call DIALS to update the dashboard

Code variation 5 of 7Related 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 6 of 7Minor and very low-impact

Tap on a version to expand it, and tap it again to show to all variations.

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, 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 7 of 7A variation in the comments only

Tap on a version to expand it, and tap it again to show to all variations.

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
RSS Feed