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Elite on the BBC Micro

Flight: Ghy, Removed [Elite-A, Docked]

Name: Ghy, Removed [Show more] Type: Subroutine Category: Flight Summary: Perform a galactic hyperspace jump Deep dive: Twisting the system seeds Galaxy and system seeds
Context: See this subroutine in context in the source code References: No direct references to this subroutine in this source file

Engage the galactic hyperdrive. Called from the hyp routine above if CTRL-H is being pressed. This routine also updates the galaxy seeds to point to the next galaxy. Using a galactic hyperdrive rotates each seed byte to the left, rolling each byte left within itself like this: 01234567 -> 12345670 to get the seeds for the next galaxy. So after 8 galactic jumps, the seeds roll round to those of the first galaxy again. We always arrive in a new galaxy at galactic coordinates (96, 96), and then find the nearest system and set that as our location. Other entry points: zZ+1 Contains an RTS
\ --- Mod: Original Acornsoft code removed: -----------> \.Ghy \ LDX GHYP \ Fetch GHYP, which tells us whether we own a galactic \ BEQ hy5 \ hyperdrive, and if it is zero, which means we don't, \ \ return from the subroutine (as hy5 contains an RTS) \ INX \ We own a galactic hyperdrive, so X is &FF, so this \ \ instruction sets X = 0 \ STX GHYP \ The galactic hyperdrive is a one-use item, so set GHYP \ \ to 0 so we no longer have one fitted \ STX FIST \ Changing galaxy also clears our criminal record, so \ \ set our legal status in FIST to 0 ("clean") \ JSR wW \ Call wW to start the hyperspace countdown \ LDX #5 \ To move galaxy, we rotate the galaxy's seeds left, so \ \ set a counter in X for the 6 seed bytes \ INC GCNT \ Increment the current galaxy number in GCNT \ LDA GCNT \ Set GCNT = GCNT mod 8, so we jump from galaxy 7 back \ AND #7 \ to galaxy 0 (shown in-game as going from galaxy 8 back \ STA GCNT \ to the starting point in galaxy 1) \.G1 \ LDA QQ21,X \ Load the X-th seed byte into A \ ASL A \ Set the C flag to bit 7 of the seed \ ROL QQ21,X \ Rotate the seed in memory, which will add bit 7 back \ \ in as bit 0, so this rolls the seed around on itself \ DEX \ Decrement the counter \ BPL G1 \ Loop back for the next seed byte, until we have \ \ rotated them all \.zZ \ LDA #96 \ Set (QQ9, QQ10) to (96, 96), which is where we always \ STA QQ9 \ arrive in a new galaxy (the selected system will be \ STA QQ10 \ set to the nearest actual system later on) \ JSR TT110 \ Call TT110 to show the front space view \ JSR TT111 \ Call TT111 to set the current system to the nearest \ \ system to (QQ9, QQ10), and put the seeds of the \ \ nearest system into QQ15 to QQ15+5 \ \ \ \ This call fixes a bug in the cassette version, where \ \ the galactic hyperdrive will take us to coordinates \ \ (96, 96) in the new galaxy, even if there isn't \ \ actually a system there, so if we jump when we are \ \ low on fuel, it is possible to get stuck in the \ \ middle of nowhere when changing galaxy \ \ \ \ This call sets the current system correctly, so we \ \ always arrive at the nearest system to (96, 96) \ LDX #0 \ Set the distance to the selected system in QQ8(1 0) \ STX QQ8 \ to 0 \ STX QQ8+1 \ LDA #116 \ Print recursive token 116 (GALACTIC HYPERSPACE ") \ JSR MESS \ as an in-flight message \ \ Fall through into jmp to set the system to the \ \ current system and return from the subroutine there \ --- End of removed code ----------------------------->