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Elite on the BBC Micro

Version analysis of IRQ1

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

Code variations between these versions are shown below.

Name: IRQ1 Type: Subroutine Category: Screen mode Summary: The main screen-mode interrupt handler (IRQ1V points here) Deep dive: The split-screen mode
The main interrupt handler, which implements Elite's split-screen mode (see the deep dive on "The split-screen mode" for details). IRQ1V is set to point to IRQ1 by the loading process.

Code variation 1 of 18A variation in the comments only

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

6502SP

Other entry points: VNT3+1 Changing this byte modifies the palette-loading instruction at VNT3, to support the #SETVDU19 <offset> command for changing the mode 1 palette

Code variation 2 of 18Related to the screen mode

This variation is blank in the 6502 Second Processor and Master versions.

Cassette, Flight

.LINSCN \ This is called from the interrupt handler below, at \ the start of each vertical sync (i.e. when the screen \ refresh starts) LDA #30 \ Set the line scan counter to a non-zero value, so STA DL \ routines like WSCAN can set DL to 0 and then wait for \ it to change to non-zero to catch the vertical sync STA VIA+&44 \ Set 6522 System VIA T1C-L timer 1 low-order counter \ (SHEILA &44) to 30 LDA #VSCAN \ Set 6522 System VIA T1C-L timer 1 high-order counter STA VIA+&45 \ (SHEILA &45) to VSCAN (57) to start the T1 counter \ counting down from 14622 at a rate of 1 MHz LDA HFX \ If HFX is non-zero, jump to VNT1 to set the mode 5 BNE VNT1 \ palette instead of switching to mode 4, which will \ have the effect of blurring and colouring the top \ screen. This is how the white hyperspace rings turn \ to colour when we do a hyperspace jump, and is \ triggered by setting HFX to 1 in routine LL164 LDA #%00001000 \ Set the Video ULA control register (SHEILA &20) to STA VIA+&20 \ %00001000, which is the same as switching to mode 4 \ (i.e. the top part of the screen) but with no cursor .VNT3 LDA TVT1+16,Y \ Copy the Y-th palette byte from TVT1+16 to SHEILA &21 STA VIA+&21 \ to map logical to actual colours for the bottom part \ of the screen (i.e. the dashboard) DEY \ Decrement the palette byte counter BPL VNT3 \ Loop back to VNT3 until we have copied all the \ palette bytes

Code variation 3 of 18Specific to an individual platform

This variation is blank in the 6502 Second Processor and Master versions.

Cassette, Flight

LDA LASCT \ Decrement the value of LASCT, but if we go too far BEQ P%+5 \ and it becomes negative, bump it back up again (this DEC LASCT \ controls the pulsing of pulse lasers)

Code variation 4 of 18Specific to an individual platform

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

Cassette

LDA SVN \ If SVN is non-zero, we are in the process of saving BNE jvec \ the commander file, so jump to jvec to pass control \ to the next interrupt handler, so we don't break file \ saving by blocking the interrupt chain

Code variation 5 of 18Minor and very low-impact

This variation is blank in the 6502 Second Processor and Master versions.

Cassette, Flight

PLA \ Otherwise restore Y from the stack TAY LDA VIA+&41 \ Read 6522 System VIA input register IRA (SHEILA &41) LDA &FC \ Set A to the interrupt accumulator save register, \ which restores A to the value it had on entering the \ interrupt RTI \ Return from interrupts, so this interrupt is not \ passed on to the next interrupt handler, but instead \ the interrupt terminates here
.IRQ1

Code variation 6 of 18Minor and very low-impact

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

Cassette, Flight, 6502SP

Master

TYA \ Store Y on the stack PHA
PHY \ Store Y on the stack

Code variation 7 of 18Related to the screen mode

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

Cassette, Flight

6502SP, Master

LDY #11 \ Set Y as a counter for 12 bytes, to use when setting \ the dashboard palette below
LDY #15 \ Set Y as a counter for 16 bytes, to use when setting \ the dashboard palette below
 LDA #%00000010         \ Read the 6522 System VIA status byte bit 1 (SHEILA
 BIT VIA+&4D            \ &4D), which is set if vertical sync has occurred on
                        \ the video system

 BNE LINSCN             \ If we are on the vertical sync pulse, jump to LINSCN
                        \ to set up the timers to enable us to switch the
                        \ screen mode between the space view and dashboard

Code variation 8 of 18Specific to an individual platform

This variation is blank in the Master version.

Cassette, Flight, 6502SP

BVC jvec \ Read the 6522 System VIA status byte bit 6, which is \ set if timer 1 has timed out. We set the timer in \ LINSCN above, so this means we only run the next bit \ if the screen redraw has reached the boundary between \ the space view and the dashboard. Otherwise bit 6 is \ clear and we aren't at the boundary, so we jump to \ jvec to pass control to the next interrupt handler

Code variation 9 of 18Related to the screen mode

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

Cassette, Flight

6502SP, Master

ASL A \ Double the value in A to 4 STA VIA+&20 \ Set the Video ULA control register (SHEILA &20) to \ %00000100, which is the same as switching to mode 5, \ (i.e. the bottom part of the screen) but with no \ cursor
LDA #%00010100 \ Set the Video ULA control register (SHEILA &20) to STA VIA+&20 \ %00010100, which is the same as switching to mode 2, \ (i.e. the bottom part of the screen) but with no \ cursor

Code variation 10 of 18Related to the screen mode

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

Cassette, Flight

6502SP, Master

LDA ESCP \ If an escape pod is fitted, jump to VNT1 to set the BNE VNT1 \ mode 5 palette differently (so the dashboard is a \ different colour if we have an escape pod) LDA TVT1,Y \ Copy the Y-th palette byte from TVT1 to SHEILA &21 STA VIA+&21 \ to map logical to actual colours for the bottom part \ of the screen (i.e. the dashboard) DEY \ Decrement the palette byte counter BPL P%-7 \ Loop back to the LDA TVT1,Y instruction until we have \ copied all the palette bytes
LDA ESCP \ Set A = ESCP, which is &FF if we have an escape pod \ fitted, or 0 if we don't AND #4 \ Set A = 4 if we have an escape pod fitted, or 0 if we \ don't EOR #&34 \ Set A = &30 if we have an escape pod fitted, or &34 if \ we don't STA &FE21 \ Store A in SHEILA &21 to map colour 3 (#YELLOW2) to \ white if we have an escape pod fitted, or yellow if we \ don't, so the outline colour of the dashboard changes \ from yellow to white if we have an escape pod fitted \ The following loop copies bytes #15 to #1 from TVT1 to \ SHEILA &21, but not byte #0, as we just did that \ colour mapping .VNT2 LDA TVT1,Y \ Copy the Y-th palette byte from TVT1 to SHEILA &21 STA &FE21 \ to map logical to actual colours for the bottom part \ of the screen (i.e. the dashboard) DEY \ Decrement the palette byte counter BNE VNT2 \ Loop back to VNT2 until we have copied all the palette \ bytes bar the first one

Code variation 11 of 18Specific to an individual platform

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

Cassette, Flight, 6502SP

Master

.jvec PLA \ Restore Y from the stack TAY JMP (VEC) \ Jump to the address in VEC, which was set to the \ original IRQ1V vector by the loading process, so this \ instruction passes control to the next interrupt \ handler
IF _COMPACT LDA MOS \ If MOS = 0 then this is a Master Compact, so jump to BEQ NOADC \ NOADC to skip reading the ADC channels (as the Compact \ has a digital joystick rather than an analogue one) ENDIF LDA VIA+&18 \ Fetch the ADC channel number into Y from bits 1-2 in AND #3 \ the ADC status byte at SHEILA &18 TAY LDA VIA+&19 \ Fetch the high byte of the value on this ADC channel \ to read the relevant joystick position STA ADCH1,Y \ Store this value in the apropriate ADCH1-ADCH3 byte INY \ Increment the channel number TYA \ If the new channel number in A < 3, skip the next two CMP #3 \ instructions BCC P%+4 LDA #0 \ Set the ADC status byte at SHEILA &18 to 0 STA VIA+&18 .NOADC PLY \ Restore Y from the stack LDA VIA+&44 \ Read 6522 System VIA T1C-L timer 1 low-order counter \ (SHEILA &44) LDA &FC \ Restore the value of A from before the call to the \ interrupt handler (the MOS stores the value of A in \ location &FC before calling the interrupt handler) RTI \ Return from interrupts, so this interrupt is not \ passed on to the next interrupt handler, but instead \ the interrupt terminates here

Code variation 12 of 18Related to the screen mode

This variation is blank in the 6502 Second Processor and Master versions.

Cassette, Flight

.VNT1 LDY #7 \ Set Y as a counter for 8 bytes LDA TVT1+8,Y \ Copy the Y-th palette byte from TVT1+8 to SHEILA &21 STA VIA+&21 \ to map logical to actual colours for the bottom part \ of the screen (i.e. the dashboard) DEY \ Decrement the palette byte counter BPL VNT1+2 \ Loop back to the LDA TVT1+8,Y instruction until we \ have copied all the palette bytes BMI jvec \ Jump up to jvec to pass control to the next interrupt \ handler (this BMI is effectively a JMP as we didn't \ loop back with the BPL above, so BMI is always true)

Code variation 13 of 18Related to the screen mode

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

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

Master

6502SP

.LINSCN LDA VIA+&41 \ Read 6522 System VIA input register IRA (SHEILA &41) LDA &FC \ Fetch the value of A from before the call to the \ interrupt handler (the MOS stores the value of A in \ location &FC before calling the interrupt handler) PHA \ Store the original value of A on the stack LDA DLCNT \ Set the line scan counter to the value of DLCNT (which STA DL \ contains 30 by default and doesn't change), so \ routines like WSCAN can set DL to 0 and then wait for \ it to change to this value to catch the vertical sync
.LINSCN \ This is called from the interrupt handler below, at \ the start of each vertical sync (i.e. when the screen \ refresh starts) LDA #30 \ Set the line scan counter to a non-zero value, so STA DL \ routines like WSCAN can set DL to 0 and then wait for \ it to change to non-zero to catch the vertical sync

Code variation 14 of 18Related to the screen mode

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

6502SP, Master

STA VIA+&44 \ Set 6522 System VIA T1C-L timer 1 low-order counter \ (SHEILA &44) to 30

Code variation 15 of 18Related to the screen mode

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

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

Master

6502SP

LDA VSCAN \ Set 6522 System VIA T1C-L timer 1 high-order counter STA VIA+&45 \ (SHEILA &45) to the contents of VSCAN (57) to start \ the T1 counter counting down from 14622 at a rate of \ 1 MHz
LDA #VSCAN \ Set 6522 System VIA T1C-L timer 1 high-order counter STA VIA+&45 \ (SHEILA &45) to VSCAN (57) to start the T1 counter \ counting down from 14622 at a rate of 1 MHz

Code variation 16 of 18Related to the screen mode

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

6502SP, Master

LDA HFX \ If the hyperspace effect flag in HFX is non-zero, then BNE jvec \ jump up to jvec to pass control to the next interrupt \ handler, instead of switching the palette to mode 1. \ This will have the effect of blurring and colouring \ the top screen in a mode 2 palette, making the \ hyperspace rings turn multicoloured when we do a \ hyperspace jump. This effect is triggered by the \ parasite issuing a #DOHFX 1 command in routine LL164 \ and is disabled again by a #DOHFX 0 command LDA #%00011000 \ Set the Video ULA control register (SHEILA &20) to STA VIA+&20 \ %00011000, which is the same as switching to mode 1 \ (i.e. the top part of the screen) but with no cursor .VNT3 \ The following instruction gets modified in-place by \ the #SETVDU19 command, which changes the \ value of TVT3+1 (i.e. the low byte of the address in \ the LDA instruction). This changes the palette block \ that gets copied to SHEILA &21, so a #SETVDU19 32 \ command applies the third palette from TVT3 in this \ loop, for example LDA TVT3,Y \ Copy the Y-th palette byte from TVT3 to SHEILA &21 STA VIA+&21 \ to map logical to actual colours for the bottom part \ of the screen (i.e. the dashboard) DEY \ Decrement the palette byte counter BNE VNT3 \ Loop back to VNT3 until we have copied all the \ palette bytes

Code variation 17 of 18Minor and very low-impact

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

6502SP

PLA \ Otherwise restore Y from the stack TAY LDA VIA+&41 \ Read 6522 System VIA input register IRA (SHEILA &41) LDA &FC \ Set A to the interrupt accumulator save register, \ which restores A to the value it had on entering the \ interrupt RTI \ Return from interrupts, so this interrupt is not \ passed on to the next interrupt handler, but instead \ the interrupt terminates here

Code variation 18 of 18Specific to an individual platform

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

Master

.jvec PHX \ Call NOISE2 to send the current sound data to the JSR NOISE2 \ 76489 sound chip, stashing X on the stack so it gets PLX \ preserved across the call PLA \ Restore A from the stack PLY \ Restore Y from the stack RTI \ Return from interrupts, so this interrupt is not \ passed on to the next interrupt handler, but instead \ the interrupt terminates here
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