Skip to navigation

Elite on the BBC Micro

Loader: Elite loader [Master version, Loader]

Name: Elite loader [Show more] Type: Subroutine Category: Loader Summary: Perform a number of OS calls, check for sideways RAM, load and move the main game data, and load and run the main game code
Context: See this subroutine in context in the source code References: No direct references to this subroutine in this source file

The loader loads and moves the following files. There is no decryption at this stage - that is all done by the main game code. * The BDATA game data file is loaded into main memory at &1300-&54FF, and is then moved as follows: * &1300-&21FF is moved to &7000-&7EFF in screen memory (i.e. shadow RAM) for the dashboard * &2200-&54FF is moved to &7F00-&B1FF in main memory, where the main game code will decrypt it * The main game code file is loaded into main memory at &1300 and the game isstarted by jumping to &2C6C The main game code file is called BCODE in the Master release and ELITE in the Master Compact release. BCODE loads into &1300-&7F47, while ELITE loads into &1300-&7FEC. The main game code is then responsible for decrypting BDATA (from &8000 to &B1FF) and BCODE/ELITE (from the end of the scramble routine to the end of the file).
.ENTRY LDA #16 \ Call OSBYTE with A = 16 and X = 0 to set the ADC to LDX #0 \ sample no channels from the joystick/Bitstik JSR OSBYTE IF _COMPACT LDA #129 \ Call OSBYTE with A = 129, X = 0 and Y = &FF to detect LDX #0 \ the machine type. This call is undocumented and is not LDY #&FF \ the recommended way to determine the machine type JSR OSBYTE \ (OSBYTE 0 is the correct way), but this call returns \ the following: \ \ * X = Y = &F5 if this is a Master Compact with MOS 5 LDA #&FF \ Set A = &FF, the value we want to store in the MOS \ flag if this is not a Master Compact CPX #&F5 \ If X <> &F5, skip the following instruction as this is BNE P%+4 \ a Master Compact LDA #0 \ This is a Master Compact, so set A = 0 STA MOS \ Store the value of A in MOS, which will be 0 if this \ is a Master Compact, or &FF if it isn't ENDIF LDA #200 \ Call OSBYTE with A = 200, X = 1 and Y = 0 to disable LDX #1 \ the ESCAPE key and disable memory clearing if the JSR OSB \ BREAK key is pressed LDA #13 \ Call OSBYTE with A = 13, X = 0 and Y = 0 to disable LDX #0 \ the "output buffer empty" event JSR OSB LDA #144 \ Call OSBYTE with A = 144, X = 255 and Y = 0 to move LDX #255 \ the screen down one line and turn screen interlace on LDY #0 JSR OSBYTE LDA #144 \ Repeat the above command, which has the effect of LDX #255 \ setting the interlace to the original value, as the JSR OSBYTE \ OSBYTE call above returns the original setting in Y LDA #225 \ Call OSBYTE with A = 225, X = 128 and Y = 0 to set LDX #128 \ the function keys to return ASCII codes for SHIFT-fn JSR OSB \ keys (i.e. add 128) LDA #13 \ Call OSBYTE with A = 13, X = 2 and Y = 0 to disable LDX #2 \ the "character entering buffer" event JSR OSB LDA #LO(B%) \ Set ZP(1 0) to point to the VDU code table at B% STA ZP LDA #HI(B%) STA ZP+1 LDY #0 \ We are now going to send the N% VDU bytes in the table \ at B% to OSWRCH to set up the special mode 1 screen \ that forms the basis for the split-screen mode .LOOP LDA (ZP),Y \ Pass the Y-th byte of the B% table to OSWRCH JSR OSWRCH INY \ Increment the loop counter CPY #N% \ Loop back for the next byte until we have done them BNE LOOP \ all (the number of bytes was set in N% above) LDA #%00001111 \ Set the Access Control latch at SHEILA &34, as STA VIA+&34 \ follows: \ \ * Bit 7 = IRR = 0: Do not IRQ the CPU with this \ * Bit 6 = TST = 0: Must be set to 0 \ * Bit 5 = IFJ = 0: &FC00-&FDFF maps to the 1Mhz bus \ * Bit 4 = ITU = 0: CPU can access external co-pro \ * Bit 3 = Y = 1: &C000-&DFFF set to 8K private RAM \ * Bit 2 = X = 1: &3000-&7FFF set to 20K shadow RAM \ * Bit 1 = E = 1: All shadow RAM locations accessible \ * Bit 0 = D = 1: Display shadow RAM as screen memory \ \ In short, this switches the screen memory, which is in \ shadow RAM, into the memory map at &3000-&7FFF, so now \ we can poke directly to the screen memory, and it also \ maps the filing system RAM space into &C000-&DFFF \ (HAZEL), in place of the MOS VDU workspace JSR PLL1 \ Call PLL1 to draw Saturn LDA #%00001001 \ Clear bits 1 and 2 of the the Access Control latch STA VIA+&34 \ at SHEILA &34, which changes the following: \ \ * Bit 2 = X = 0: &3000-&7FFF set to main RAM \ * Bit 1 = E = 0: VDU shadow RAM locations accessible \ \ In short, this switches the screen memory, which is in \ shadow RAM, out of the memory map, so &3000-&7FFF is \ now mapped to main RAM and we can't update the screen LDA #4 \ Call OSBYTE with A = 4, X = 1 and Y = 0 to disable LDX #1 \ cursor editing, so the cursor keys return ASCII values JSR OSB \ and can therefore be used in-game LDA #9 \ Call OSBYTE with A = 9, X = 0 and Y = 0 to disable LDX #0 \ flashing colours JSR OSB LDX #LO(MESS1) \ Set (Y X) to point to MESS1 ("L.BDATA FFFF1300") LDY #HI(MESS1) JSR OSCLI \ Call OSCLI to run the OS command in MESS1, which \ loads the BDATA file to address &1300-&54FF, appending \ &FFFF to the address to make sure it loads in the main \ BBC Master rather than getting passed across the Tube \ to the Second Processor, if one is fitted LDA #6 \ Set the RAM copy of the currently selected paged ROM STA LATCH \ to 6, so it matches the paged ROM selection latch at \ SHEILA &30 that we are about to set LDA VIA+&30 \ Set bits 0-3 of the ROM Select latch at SHEILA &30 to AND #%11110000 \ 6, to switch sideways RAM bank 6 into into &8000-&BFFF ORA #6 \ in main memory STA VIA+&30 LDA #%10101010 \ Set A and location &8000 to %10101010 STA &8000 LSR A \ Shift A and location &8000 right LSR &8000 CMP &8000 \ If A matches location &8000 (i.e. both now contain BEQ OK \ %01010101) then jump to OK, as ROM bank 6 is writable \ and does indeed contain sideways RAM rather than a \ paged ROM, which is what we need for running the game BRK \ Otherwise we can't run the game, so terminate the \ loader with the following error message EQUB 0 \ Error number EQUB 22, 7 \ Switch to mode 7 and clear the screen EQUS "ELITE needs RAM in slot #6" EQUB 0 \ End of error message .OK LDA #%00001111 \ Set bits 1 and 2 of the Access Control Register at STA VIA+&34 \ SHEILA &34 to switch screen memory into &3000-&7FFF \ We now want to copy &F pages of memory (&F00 bytes) \ from &1300-&21FF to &7000-&7EFF in screen memory LDX #&F \ Set a page counter in X to copy &F pages LDA #&13 \ Set ZP(1 0) = &1300 STA ZP+1 STZ ZP STZ P \ Set P(1 0) = &7000 LDA #&70 STA P+1 LDY #0 \ Set Y = 0 to act as a byte counter within each page .MPL1 LDA (ZP),Y \ Copy the Y-th byte of the memory block at ZP(1 0) to STA (P),Y \ the Y-th byte of the memory block at P(1 0) DEY \ Decrement the byte counter BNE MPL1 \ Loop back to copy the next byte until we have copied a \ whole page of 256 bytes INC ZP+1 \ Increment the high bytes of both ZP(1 0) and P(1 0) INC P+1 \ so we copy the next page in memory DEX \ Decrement the page counter BNE MPL1 \ Loop back to copy the next page until we have done all \ &F of them LDA #%00001001 \ Clear bits 1 and 2 of the Access Control Register at STA VIA+&34 \ SHEILA &34 to switch main memory back into &3000-&7FFF \ We now want to copy &33 pages of memory (&3300 bytes) \ from &2200-&54FF to &7F00-&B1FF in main memory LDX #&33 \ Set a page counter in X to copy &33 pages .MPL2 LDA (ZP),Y \ Copy the Y-th byte of the memory block at ZP(1 0) to STA (P),Y \ the Y-th byte of the memory block at P(1 0) DEY \ Decrement the byte counter BNE MPL2 \ Loop back to copy the next byte until we have copied a \ whole page of 256 bytes INC ZP+1 \ Increment the high bytes of both ZP(1 0) and P(1 0) INC P+1 \ so we copy the next page in memory DEX \ Decrement the page counter BNE MPL2 \ Loop back to copy the next page until we have done all \ &33 of them CLI \ Enable interrupts LDX #LO(MESS2) \ Set (Y X) to point to MESS2 ("L.BCODE FFFF1300" in the LDY #HI(MESS2) \ Master release, or "L.ELITE FFFF1300" in the Master \ Compact release) JSR OSCLI \ Call OSCLI to run the OS command in MESS2, which loads \ the BCODE/ELITE file to address &1300-&7F48, appending \ &FFFF to the address to make sure it loads in the main \ BBC Master rather than getting passed across the Tube \ to the Second Processor, if one is fitted LDX #LO(MESS3) \ Set (Y X) to point to MESS3 ("DIR E") LDY #HI(MESS3) JSR OSCLI \ Call OSCLI to run the OS command in MESS3, which \ changes the disc directory to E LDA #6 \ Set the RAM copy of the currently selected paged ROM STA LATCH \ to 6, so it matches the paged ROM selection latch at \ SHEILA &30 that we are about to set LDA VIA+&30 \ Switch ROM bank 6 into memory by setting bits 0-3 of AND #%11110000 \ the ROM selection latch at SHEILA &30 to 6 ORA #6 STA VIA+&30 JMP &2C6C \ Jump to the start of the main game code at &2C6C, \ which we just loaded in the BCODE/ELITE file