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Elite on the BBC Micro

Flight: Ghy [6502SP version]

Name: Ghy [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 Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * hyp calls Ghy

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
.Ghy LDX GHYP \ Fetch GHYP, which tells us whether we own a galactic BEQ zZ+1 \ hyperdrive, and if it is zero, which means we don't, \ return from the subroutine (as zZ+1 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") LDA #2 \ Call wW2 with A = 2 to start the hyperspace countdown, JSR wW2 \ but starting the countdown from 2 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 #5 \ We now want to copy those seeds into safehouse, so we \ so set a counter in Xto copy 6 bytes .dumdeedum LDA QQ15,X \ Copy the X-th byte of QQ15 into the X-th byte of STA safehouse,X \ safehouse DEX \ Decrement the loop counter BPL dumdeedum \ Loop back to copy the next byte until we have copied \ all six seed bytes 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