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BBC Micro Elite

Workspaces: ZP

Name: ZP [View in context] Type: Workspace Address: &0000 to &00B0 Category: Workspaces Summary: Lots of important variables are stored in the zero page workspace as it is quicker and more space-efficient to access memory here Deep dive: The Elite memory map
ORG &0000 .ZP SKIP 0 \ The start of the zero page workspace .RAND SKIP 4 \ Four 8-bit seeds for the random number generation \ system implemented in the DORND routine .TRTB% SKIP 2 \ This is set by elite-loader.asm to point to the MOS \ keyboard translation table, which is used by the TT217 \ routine to translate internal key values to ASCII .T1 SKIP 1 \ Temporary storage, used in a number of places .SC SKIP 1 \ Screen address (low byte) \ \ Elite draws on-screen by poking bytes directly into \ screen memory, and SC(1 0) is typically set to the \ address of the character block containing the pixel \ we want to draw (see the deep dive on "Drawing \ monochrome pixels in mode 4" for more details) .SCH SKIP 1 \ Screen address (high byte) .XX16 SKIP 18 \ Temporary storage for a block of values, used in a \ number of places .P SKIP 3 \ Temporary storage, used in a number of places .XX0 SKIP 2 \ Temporary storage, used to store the address of a ship \ blueprint. For example, it is used when we add a new \ ship to the local bubble in routine NWSHP, and it \ contains the address of the current ship's blueprint \ as we loop through all the nearby ships in the main \ flight loop .INF SKIP 2 \ Temporary storage, typically used for storing the \ address of a ship's data block, so it can be copied \ to and from the internal workspace at INWK .V SKIP 2 \ Temporary storage, typically used for storing an \ address pointer .XX SKIP 2 \ Temporary storage, typically used for storing a 16-bit \ x-coordinate .YY SKIP 2 \ Temporary storage, typically used for storing a 16-bit \ y-coordinate .SUNX SKIP 2 \ The 16-bit x-coordinate of the vertical centre axis \ of the sun (which might be off-screen) .BETA SKIP 1 \ The current pitch angle beta, which is reduced from \ JSTY to a sign-magnitude value between -8 and +8 \ \ This describes how fast we are pitching our ship, and \ determines how fast the universe pitches around us \ \ The sign bit is also stored in BET2, while the \ opposite sign is stored in BET2+1 .BET1 SKIP 1 \ The magnitude of the pitch angle beta, i.e. |beta|, \ which is a positive value between 0 and 8 .XC SKIP 1 \ The x-coordinate of the text cursor (i.e. the text \ column), which can be from 0 to 32 \ \ A value of 0 denotes the leftmost column and 32 the \ rightmost column, but because the top part of the \ screen (the mode 4 part) has a white border that \ clashes with columns 0 and 32, text is only shown \ in columns 1-31 .YC SKIP 1 \ The y-coordinate of the text cursor (i.e. the text \ row), which can be from 0 to 23 \ \ The screen actually has 31 character rows if you \ include the mode 5 dashboard, but the text printing \ routines only work on the mode 4 part (the space \ view), so the text cursor only goes up to a maximum of \ 23, the row just before the screen split \ \ A value of 0 denotes the top row, but because the \ top part of the screen has a white border that clashes \ with row 0, text is always shown at row 1 or greater .QQ22 SKIP 2 \ The two hyperspace countdown counters \ \ Before a hyperspace jump, both QQ22 and QQ22+1 are \ set to 15 \ \ QQ22 is an internal counter that counts down by 1 \ each time TT102 is called, which happens every \ iteration of the main game loop. When it reaches \ zero, the on-screen counter in QQ22+1 gets \ decremented, and QQ22 gets set to 5 and the countdown \ continues (so the first tick of the hyperspace counter \ takes 15 iterations to happen, but subsequent ticks \ take 5 iterations each) \ \ QQ22+1 contains the number that's shown on-screen \ during the countdown. It counts down from 15 to 1, and \ when it hits 0, the hyperspace engines kick in .ECMA SKIP 1 \ The E.C.M. countdown timer, which determines whether \ an E.C.M. system is currently operating: \ \ * 0 = E.C.M. is off \ \ * Non-zero = E.C.M. is on and is counting down \ \ The counter starts at 32 when an E.C.M. is activated, \ either by us or by an opponent, and it decreases by 1 \ in each iteration of the main flight loop until it \ reaches zero, at which point the E.C.M. switches off. \ Only one E.C.M. can be active at any one time, so \ there is only one counter .XX15 SKIP 0 \ Temporary storage, typically used for storing screen \ coordinates in line-drawing routines \ \ There are six bytes of storage, from XX15 TO XX15+5. \ The first four bytes have the following aliases: \ \ X1 = XX15 \ Y1 = XX15+1 \ X2 = XX15+2 \ Y2 = XX15+3 \ \ These are typically used for describing lines in terms \ of screen coordinates, i.e. (X1, Y1) to (X2, Y2) \ \ The last two bytes of XX15 do not have aliases .X1 SKIP 1 \ Temporary storage, typically used for x-coordinates in \ line-drawing routines .Y1 SKIP 1 \ Temporary storage, typically used for y-coordinates in \ line-drawing routines .X2 SKIP 1 \ Temporary storage, typically used for x-coordinates in \ line-drawing routines .Y2 SKIP 1 \ Temporary storage, typically used for y-coordinates in \ line-drawing routines SKIP 2 \ The last 2 bytes of the XX15 block .XX12 SKIP 6 \ Temporary storage for a block of values, used in a \ number of places .K SKIP 4 \ Temporary storage, used in a number of places .KL SKIP 1 \ The following bytes implement a key logger that \ enables Elite to scan for concurrent key presses of \ the primary flight keys, plus a secondary flight key \ \ See the deep dive on "The key logger" for more details \ \ If a key is being pressed that is not in the keyboard \ table at KYTB, it can be stored here (as seen in \ routine DK4, for example) .KY1 SKIP 1 \ "?" has been pressed \ \ * 0 = no \ \ * Non-zero = yes .KY2 SKIP 1 \ Space has been pressed \ \ * 0 = no \ \ * Non-zero = yes .KY3 SKIP 1 \ "<" has been pressed \ \ * 0 = no \ \ * Non-zero = yes .KY4 SKIP 1 \ ">" has been pressed \ \ * 0 = no \ \ * Non-zero = yes .KY5 SKIP 1 \ "X" has been pressed \ \ * 0 = no \ \ * Non-zero = yes .KY6 SKIP 1 \ "S" has been pressed \ \ * 0 = no \ \ * Non-zero = yes .KY7 SKIP 1 \ "A" has been pressed \ \ * 0 = no \ \ * Non-zero = yes \ \ This is also set when the joystick fire button has \ been pressed .KY12 SKIP 1 \ Tab key has been pressed \ \ * 0 = no \ \ * Non-zero = yes .KY13 SKIP 1 \ Escape key has been pressed \ \ * 0 = no \ \ * Non-zero = yes .KY14 SKIP 1 \ "T" has been pressed \ \ * 0 = no \ \ * Non-zero = yes .KY15 SKIP 1 \ "U" has been pressed \ \ * 0 = no \ \ * Non-zero = yes .KY16 SKIP 1 \ "M" has been pressed \ \ * 0 = no \ \ * Non-zero = yes .KY17 SKIP 1 \ "E" has been pressed \ \ * 0 = no \ \ * Non-zero = yes .KY18 SKIP 1 \ "J" has been pressed \ \ * 0 = no \ \ * Non-zero = yes .KY19 SKIP 1 \ "C" has been pressed \ \ * 0 = no \ \ * Non-zero = yes .LAS SKIP 1 \ Contains the laser power of the laser fitted to the \ current space view (or 0 if there is no laser fitted \ to the current view) \ \ This gets set to bits 0-6 of the laser power byte from \ the commander data block, which contains the laser's \ power (bit 7 doesn't denote laser power, just whether \ or not the laser pulses, so that is not stored here) .MSTG SKIP 1 \ The current missile lock target \ \ * &FF = no target \ \ * 1-13 = the slot number of the ship that our \ missile is locked onto .XX1 SKIP 0 \ This is an alias for INWK that is used in the main \ ship-drawing routine at LL9 .INWK SKIP 33 \ The zero-page internal workspace for the current ship \ data block \ \ As operations on zero page locations are faster and \ have smaller opcodes than operations on the rest of \ the addressable memory, Elite tends to store oft-used \ data here. A lot of the routines in Elite need to \ access and manipulate ship data, so to make this an \ efficient exercise, the ship data is first copied from \ the ship data blocks at K% into INWK (or, when new \ ships are spawned, from the blueprints at XX21). See \ the deep dive on "Ship data blocks" for details of \ what each of the bytes in the INWK data block \ represents .XX19 SKIP NI% - 33 \ XX19(1 0) shares its location with INWK(34 33), which \ contains the address of the ship line heap .LSP SKIP 1 \ The ball line heap pointer, which contains the number \ of the first free byte after the end of the LSX2 and \ LSY2 heaps (see the deep dive on "The ball line heap" \ for details) .QQ15 SKIP 6 \ The three 16-bit seeds for the selected system, i.e. \ the one in the crosshairs in the Short-range Chart \ \ See the deep dives on "Galaxy and system seeds" and \ "Twisting the system seeds" for more details .K5 SKIP 0 \ Temporary storage used to store segment coordinates \ across successive calls to BLINE, the ball line \ routine .XX18 SKIP 0 \ Temporary storage used to store coordinates in the \ LL9 ship-drawing routine .QQ17 SKIP 1 \ Contains a number of flags that affect how text tokens \ are printed, particularly capitalisation: \ \ * If all bits are set (255) then text printing is \ disabled \ \ * Bit 7: 0 = ALL CAPS \ 1 = Sentence Case, bit 6 determines the \ case of the next letter to print \ \ * Bit 6: 0 = print the next letter in upper case \ 1 = print the next letter in lower case \ \ * Bits 0-5: If any of bits 0-5 are set, print in \ lower case \ \ So: \ \ * QQ17 = 0 means case is set to ALL CAPS \ \ * QQ17 = %10000000 means Sentence Case, currently \ printing upper case \ \ * QQ17 = %11000000 means Sentence Case, currently \ printing lower case \ \ * QQ17 = %11111111 means printing is disabled .QQ19 SKIP 3 \ Temporary storage, used in a number of places .K6 SKIP 5 \ Temporary storage, typically used for storing \ coordinates during vector calculations .ALP1 SKIP 1 \ Magnitude of the roll angle alpha, i.e. |alpha|, \ which is a positive value between 0 and 31 .ALP2 SKIP 2 \ Bit 7 of ALP2 = sign of the roll angle in ALPHA \ \ Bit 7 of ALP2+1 = opposite sign to ALP2 and ALPHA .BET2 SKIP 2 \ Bit 7 of BET2 = sign of the pitch angle in BETA \ \ Bit 7 of BET2+1 = opposite sign to BET2 and BETA .DELTA SKIP 1 \ Our current speed, in the range 1-40 .DELT4 SKIP 2 \ Our current speed * 64 as a 16-bit value \ \ This is stored as DELT4(1 0), so the high byte in \ DELT4+1 therefore contains our current speed / 4 .U SKIP 1 \ Temporary storage, used in a number of places .Q SKIP 1 \ Temporary storage, used in a number of places .R SKIP 1 \ Temporary storage, used in a number of places .S SKIP 1 \ Temporary storage, used in a number of places .XSAV SKIP 1 \ Temporary storage for saving the value of the X \ register, used in a number of places .YSAV SKIP 1 \ Temporary storage for saving the value of the Y \ register, used in a number of places .XX17 SKIP 1 \ Temporary storage, used in BPRNT to store the number \ of characters to print, and as the edge counter in the \ main ship-drawing routine .QQ11 SKIP 1 \ The number of the current view: \ \ 0 = Space view \ 1 = Title screen \ Buy Cargo screen (red key f1) \ Data on System screen (red key f6) \ Get commander name ("@", save/load commander) \ In-system jump just arrived ("J") \ Mis-jump just arrived (witchspace) \ 4 = Sell Cargo screen (red key f2) \ 6 = Death screen \ 8 = Status Mode screen (red key f8) \ Inventory screen (red key f9) \ 16 = Market Price screen (red key f7) \ 32 = Equip Ship screen (red key f3) \ 64 = Long-range Chart (red key f4) \ 128 = Short-range Chart (red key f5) \ \ This value is typically set by calling routine TT66 .ZZ SKIP 1 \ Temporary storage, typically used for distance values .XX13 SKIP 1 \ Temporary storage, typically used in the line-drawing \ routines .MCNT SKIP 1 \ The main loop counter \ \ This counter determines how often certain actions are \ performed within the main loop. See the deep dive on \ "Scheduling tasks with the main loop counter" for more \ details .DL SKIP 1 \ Vertical sync flag \ \ DL gets set to 30 every time we reach vertical sync on \ the video system, which happens 50 times a second \ (50Hz). The WSCAN routine uses this to pause until the \ vertical sync, by setting DL to 0 and then monitoring \ its value until it changes to 30 .TYPE SKIP 1 \ The current ship type \ \ This is where we store the current ship type for when \ we are iterating through the ships in the local bubble \ as part of the main flight loop. See the table at XX21 \ for information about ship types .JSTX SKIP 1 \ Our current roll rate \ \ This value is shown in the dashboard's RL indicator, \ and determines the rate at which we are rolling \ \ The value ranges from from 1 to 255 with 128 as the \ centre point, so 1 means roll is decreasing at the \ maximum rate, 128 means roll is not changing, and \ 255 means roll is increasing at the maximum rate \ \ This value is updated by "<" and ">" key presses, or \ if joysticks are enabled, from the joystick. If \ keyboard damping is enabled (which it is by default), \ the value is slowly moved towards the centre value of \ 128 (no roll) if there are no key presses or joystick \ movement .JSTY SKIP 1 \ Our current pitch rate \ \ This value is shown in the dashboard's DC indicator, \ and determines the rate at which we are pitching \ \ The value ranges from from 1 to 255 with 128 as the \ centre point, so 1 means pitch is decreasing at the \ maximum rate, 128 means pitch is not changing, and \ 255 means pitch is increasing at the maximum rate \ \ This value is updated by "S" and "X" key presses, or \ if joysticks are enabled, from the joystick. If \ keyboard damping is enabled (which it is by default), \ the value is slowly moved towards the centre value of \ 128 (no pitch) if there are no key presses or joystick \ movement .ALPHA SKIP 1 \ The current roll angle alpha, which is reduced from \ JSTX to a sign-magnitude value between -31 and +31 \ \ This describes how fast we are rolling our ship, and \ determines how fast the universe rolls around us \ \ The sign bit is also stored in ALP2, while the \ opposite sign is stored in ALP2+1 .QQ12 SKIP 1 \ Our "docked" status \ \ * 0 = we are not docked \ \ * &FF = we are docked .TGT SKIP 1 \ Temporary storage, typically used as a target value \ for counters when drawing explosion clouds and partial \ circles .SWAP SKIP 1 \ Temporary storage, used to store a flag that records \ whether or not we had to swap a line's start and end \ coordinates around when clipping the line in routine \ LL145 (the flag is used in places like BLINE to swap \ them back) .COL SKIP 1 \ Temporary storage, used to store colour information \ when drawing pixels in the mode 5 dashboard screen .FLAG SKIP 1 \ A flag that's used to define whether this is the first \ call to the ball line routine in BLINE, so it knows \ whether to wait for the second call before storing \ segment data in the ball line heap .CNT SKIP 1 \ Temporary storage, typically used for storing the \ number of iterations required when looping .CNT2 SKIP 1 \ Temporary storage, used in the planet-drawing routine \ to store the segment number where the arc of a partial \ circle should start .STP SKIP 1 \ The step size for drawing circles \ \ Circles in Elite are split up into 64 points, and the \ step size determines how many points to skip with each \ straight-line segment, so the smaller the step size, \ the smoother the circle. The values used are: \ \ * 2 for big planets and the circles on the charts \ * 4 for medium planets and the launch tunnel \ * 8 for small planets and the hyperspace tunnel \ \ As the step size increases we move from smoother \ circles at the top to more polygonal at the bottom. \ See the CIRCLE2 routine for more details .XX4 SKIP 1 \ Temporary storage, used in a number of places .XX20 SKIP 1 \ Temporary storage, used in a number of places .XX14 SKIP 1 \ This byte is unused .RAT SKIP 1 \ Used to store different signs depending on the current \ space view, for use in calculating stardust movement .RAT2 SKIP 1 \ Temporary storage, used to store the pitch and roll \ signs when moving objects and stardust .K2 SKIP 4 \ Temporary storage, used in a number of places ORG &D1 .T SKIP 1 \ Temporary storage, used in a number of places .K3 SKIP 0 \ Temporary storage, used in a number of places .XX2 SKIP 14 \ Temporary storage, used to store the visibility of the \ ship's faces during the ship-drawing routine at LL9 .K4 SKIP 2 \ Temporary storage, used in a number of places PRINT "Zero page variables from ", ~ZP, " to ", ~P%