Skip to navigation

Elite on the BBC Micro

Version analysis of TT17

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

Code variations between these versions are shown below.

Name: TT17 Type: Subroutine Category: Keyboard

Code variation 1 of 18A 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

Summary: Scan the keyboard for cursor key or joystick movement
Summary: Scan the keyboard for cursor key movement

Code variation 2 of 18A 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

Scan the keyboard and joystick for cursor key or stick movement, and return the result as deltas (changes) in x- and y-coordinates as follows: * For joystick, X and Y are integers between -2 and +2 depending on how far the stick has moved * For keyboard, X and Y are integers between -1 and +1 depending on which keys are pressed Returns: A The key pressed, if the arrow keys were used X Change in the x-coordinate according to the cursor keys being pressed or joystick movement, as an integer (see above) Y Change in the y-coordinate according to the cursor keys being pressed or joystick movement, as an integer (see above)
Scan the keyboard for cursor key movement, and return the result as deltas (changes) in x- and y-coordinates as follows: * X and Y are integers between -1 and +1 depending on which keys are pressed Returns: A The key pressed, if the arrow keys were used X Change in the x-coordinate according to the cursor keys being pressed, as an integer (see above) Y Change in the y-coordinate according to the cursor keys being pressed, as an integer (see above)
.TT17

Code variation 3 of 18Related to the Master version

The Master version doesn't check for SHIFT being held down in the space view, as it has no effect there (it's only used in the chart views for speeding up the crosshairs).

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

Master

LDA QQ11 \ If this not the space view, skip the following three BNE P%+7 \ instructions to move onto the SHIFT key logic JSR DOKEY \ This is the space view, so scan the keyboard for \ flight controls and pause keys, (or the equivalent on \ joystick) and update the key logger, setting KL to the \ key pressed TXA \ Transfer the value of the key pressed from X to A RTS \ Return from the subroutine

Code variation 4 of 18A 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 DOKEY \ Scan the keyboard for flight controls and pause keys, \ (or the equivalent on joystick) and update the key \ logger, setting KL to the key pressed
JSR DOKEY \ Scan the keyboard for flight controls and pause keys, \ and update the key logger, setting KL to the key \ pressed

Code variation 5 of 18Related to an enhanced feature

In the enhanced versions, the cursor moves more quickly in the chart views if you hold down SHIFT.

See below for more variations related to this code.

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

6502SP

LDX #0 \ Call DKS4 to check whether the SHIFT key is being JSR DKS4 \ pressed STA newlocn \ Store the result (which will have bit 7 set if SHIFT \ is being pressed) in newlocn

Code variation 6 of 18Related to the Master version

The Master Compact release allows you to move the chart crosshairs using the digital joystick, using the TT17X and DJOY routines.

See below for more variations related to this code.

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

Master

IF _COMPACT LDX #0 \ Set the initial values for the results, X = Y = 0, LDY #0 \ which we now increase or decrease appropriately ENDIF

Code variation 7 of 18Related to the Electron version

Despite never reading the joystick values from the ADC channels, the Electron version still lets the joystick control the crosshairs on the chart views, if joysticks are configured. The result is an uncontrollable crosshair that moves of its own accord, so presumably this is a bug.

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

Cassette, Flight, Docked, 6502SP, Master

Electron

LDA JSTK \ If the joystick is not configured, jump down to TJ1, BEQ TJ1 \ otherwise we move the cursor with the joystick
LDX JSTK \ If the joystick is not configured, jump down to TJ1, BEQ TJ1 \ otherwise keep going... though as the DOKEY routine \ doesn't read the ADC channels in the Electron version, \ this doesn't actually work, but instead moves the \ crosshairs in an uncontrollable way, so this is \ presumably a bug

Code variation 8 of 18Related to the Master version

See variation 6 above for details.

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

Master

IF _COMPACT LDA MOS \ If MOS = 0 then this is a Master Compact, so jump to BEQ DJOY \ DJOY to read the digital joystick before rejoining the \ routine below at TJ1 ENDIF LDA JSTY \ Fetch the joystick pitch, ranging from 1 to 255 with \ 128 as the centre point JSR TJS1 \ Call TJS1 just below to set A to a value between -4 \ and +4 depending on the joystick pitch value (moving \ the stick up and down) TAY \ Copy the result into Y
 LDA JSTX               \ Fetch the joystick roll, ranging from 1 to 255 with
                        \ 128 as the centre point

 EOR #&FF               \ Flip the sign so A = -JSTX, because the joystick roll
                        \ works in the opposite way to moving a cursor on-screen
                        \ in terms of left and right

Code variation 9 of 18A variation in the labels only

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

Cassette, Flight, Docked, 6502SP, Electron

Master

JSR TJS1 \ Call TJS1 just below to set A to a value between -2 \ and +2 depending on the joystick roll value (moving \ the stick sideways)
JSR TJS1 \ Call TJS1 just below to set A to a value between -4 \ and +4 depending on the joystick roll value (moving \ the stick sideways)

Code variation 10 of 18Related to the Master version

The Master has different logic around moving the crosshairs on the chart views, though the results appear to be the same.

See below for more variations related to this code.

This variation is blank in the Master version.

Cassette, Flight, Docked, 6502SP, Electron

TYA \ Copy Y to A

Code variation 11 of 18Related to an enhanced feature

See variation 5 above for details.

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

6502SP

BIT newlocn \ If bit 7 of newlocn is clear - in other words, if BPL P%+3 \ SHIFT is not being pressed - then skip the following \ instruction ASL A \ SHIFT is being held down, so double the value of A \ (i.e. SHIFT moves the cursor at double the speed \ when using the joystick)

Code variation 12 of 18Related to the Master version

See variation 10 above for details.

This variation is blank in the Master version.

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

Cassette

Flight, Docked, 6502SP, Electron

TAX \ Copy A to X, so X contains the joystick roll value LDA JSTY \ Fetch the joystick pitch, ranging from 1 to 255 with \ 128 as the centre point, and fall through into TJS1 to \ set Y to the joystick pitch value (moving the stick up \ and down) .TJS1 TAY \ Store A in Y LDA #0 \ Set the result, A = 0 CPY #16 \ If Y >= 16 set the C flag, so A = A - 1 SBC #0 \CPY #&20 \ These instructions are commented out in the original \SBC #0 \ source, but they would make the joystick move the \ cursor faster by increasing the range of Y by -1 to +1 CPY #64 \ If Y >= 64 set the C flag, so A = A - 1 SBC #0 CPY #192 \ If Y >= 192 set the C flag, so A = A + 1 ADC #0 CPY #224 \ If Y >= 224 set the C flag, so A = A + 1 ADC #0 \CPY #&F0 \ These instructions are commented out in the original \ADC #0 \ source, but they would make the joystick move the \ cursor faster by increasing the range of Y by -1 to +1
TAX \ Copy A to X, so X contains the joystick roll value LDA JSTY \ Fetch the joystick pitch, ranging from 1 to 255 with \ 128 as the centre point, and fall through into TJS1 to \ set Y to the joystick pitch value (moving the stick up \ and down) .TJS1 TAY \ Store A in Y LDA #0 \ Set the result, A = 0 CPY #16 \ If Y >= 16 set the C flag, so A = A - 1 SBC #0 CPY #64 \ If Y >= 64 set the C flag, so A = A - 1 SBC #0 CPY #192 \ If Y >= 192 set the C flag, so A = A + 1 ADC #0 CPY #224 \ If Y >= 224 set the C flag, so A = A + 1 ADC #0

Code variation 13 of 18Related to an enhanced feature

See variation 5 above for details.

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

6502SP

BIT newlocn \ If bit 7 of newlocn is clear - in other words, if BPL P%+3 \ SHIFT is not being pressed - then skip the following \ instruction ASL A \ SHIFT is being held down, so double the value of A \ (i.e. SHIFT moves the cursor at double the speed \ when using the joystick

Code variation 14 of 18Related to the Master version

See variation 10 above for details.

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

Cassette, Flight, Docked, 6502SP, Electron

Master

TAY \ Copy the value of A into Y LDA KL \ Set A to the value of KL (the key pressed) RTS \ Return from the subroutine
TAX \ Copy the value of A into X IF _SNG47 LDA KL \ Set A to the value of KL (the key pressed) RTS \ Return from the subroutine ENDIF

Code variation 15 of 18Related to an enhanced feature

See variation 5 above for details.

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

6502SP

.newlocn EQUB 0 \ The current key press is stored here in the above code \ when we check whether SHIFT is being held down
.TJ1

Code variation 16 of 18Specific to an individual platform

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

Cassette, Flight, Docked, 6502SP

Master

Electron

LDA KL \ Set A to the value of KL (the key pressed) LDX #0 \ Set the initial values for the results, X = Y = 0, LDY #0 \ which we now increase or decrease appropriately CMP #&19 \ If left arrow was pressed, set X = X - 1 BNE P%+3 DEX CMP #&79 \ If right arrow was pressed, set X = X + 1 BNE P%+3 INX CMP #&39 \ If up arrow was pressed, set Y = Y + 1 BNE P%+3 INY CMP #&29 \ If down arrow was pressed, set Y = Y - 1 BNE P%+3 DEY
LDA KL \ Set A to the value of KL (the key pressed) IF _SNG47 LDX #0 \ Set the results, X = Y = 0 LDY #0 ENDIF CMP #&8C \ If left arrow was pressed, set X = X - 1 BNE P%+3 DEX CMP #&8D \ If right arrow was pressed, set X = X + 1 BNE P%+3 INX CMP #&8E \ If down arrow was pressed, set Y = Y - 1 BNE P%+3 DEY CMP #&8F \ If up arrow was pressed, set Y = Y + 1 BNE P%+3 INY
LDA KL \ Set A to the value of KL (the key pressed) LDY #0 \ Set the result, Y = 0 (and we know that X is 0 as well \ as we jumped to TJ1 from above following a LDX and a \ BEQ) CMP #&18 \ If left arrow was pressed, set X = X - 1 BNE P%+3 DEX CMP #&78 \ If right arrow was pressed, set X = X + 1 BNE P%+3 INX CMP #&39 \ If up arrow was pressed, set Y = Y + 1 BNE P%+3 INY CMP #&28 \ If down arrow was pressed, set Y = Y - 1 BNE P%+3 DEY

Code variation 17 of 18Related to an enhanced feature

See variation 5 above for details.

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

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

Docked

Master

6502SP

STX T \ Set T to the value of X, which contains the joystick \ roll value LDX #0 \ Scan the keyboard to see if the SHIFT key is currently JSR DKS4 \ being pressed, returning the result in A and X BPL TJe \ If SHIFT is not being pressed, skip to TJe ASL T \ SHIFT is being held down, so quadruple the value of T ASL T \ (i.e. SHIFT moves the cursor at four times the speed \ when using the joystick) TYA \ Fetch the joystick pitch value from Y into A ASL A \ SHIFT is being held down, so quadruple the value of A ASL A \ (i.e. SHIFT moves the cursor at four times the speed \ when using the joystick) TAY \ Transfer the amended value of A back into Y .TJe LDX T \ Fetch the amended value of T back into X LDA KL \ Set A to the value of KL (the key pressed)
PHX \ Store X (which contains the change in the \ x-coordinate) on the stack so we can retrieve it later IF _SNG47 LDA #0 \ Call DKS4 to check whether the SHIFT key is being JSR DKS4 \ pressed ELIF _COMPACT LDA #0 \ Call DKS4mc to check whether the SHIFT key is being JSR DKS4mc \ pressed ENDIF BMI P%+6 \ If SHIFT is being pressed, skip the next three \ instructions PLX \ SHIFT is not being pressed, so retrieve the value of X \ we stored above so we can return it LDA KL \ Set A to the value of KL (the key pressed) RTS \ Return from the subroutine PLA \ Pull the value of X from the stack into A, so A now \ contains the change in the x-coordinate ASL A \ SHIFT is being held down, so quadruple the value of A ASL A \ (i.e. SHIFT moves the cursor at four times the speed \ when using the joystick) TAX \ Put the amended value of A back into X TYA \ Now to do the same with the change in y-coordinate, so \ fetch the value of Y into A ASL A \ SHIFT is being held down, so quadruple the value of A ASL A \ (i.e. SHIFT moves the cursor at four times the speed \ when using the joystick) TAY \ Put the amended value of A back into Y IF _COMPACT STZ KY7 \ Clear the key logger at KY7 to reset the "A" (fire) \ button to "not pressed" ENDIF LDA KL \ Set A to the value of KL (the key pressed)
TXA \ Transfer the value of X into A BIT newlocn \ If bit 7 of newlocn is clear - in other words, if BPL P%+4 \ SHIFT is not being pressed - then skip the following \ two instructions ASL A \ SHIFT is being held down, so quadruple the value of A ASL A \ (i.e. SHIFT moves the cursor at four times the speed \ when using the keyboard) TAX \ Transfer the amended value of A back into X TYA \ Transfer the value of Y into A BIT newlocn \ If bit 7 of newlocn is clear - in other words, if BPL P%+4 \ SHIFT is not being pressed - then skip the following \ two instructions ASL A \ SHIFT is being held down, so quadruple the value of A ASL A \ (i.e. SHIFT moves the cursor at four times the speed \ when using the keyboard) TAY \ Transfer the amended value of A back into Y LDA KL \ Set A to the value of KL (the key pressed)
 RTS                    \ Return from the subroutine

Code variation 18 of 18Related to the Master version

See variation 10 above for details.

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

Master

.TJS1 \ This routine calculates the following: \ \ A = round(A / 16) - 4 \ \ This set A to a value between -4 and +4, given an \ initial value ranging from 1 to 255 with 128 as \ the centre point LSR A \ Set A = A / 16 LSR A \ LSR A \ and C contains the last bit to be shifted out LSR A LSR A ADC #0 \ If that last bit was a 1, this increments A, so \ this effectively implements a rounding function, \ where 0.5 and above get rounded up SBC #3 \ The addition will not overflow, so the C flag is \ clear at this point, so this performs: \ \ A = A - 3 - (1 - C) \ = A - 3 - (1 - 0) \ = A - 3 - 1 \ = A - 4 RTS \ Return from the subroutine
RSS Feed