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BBC Micro Elite

Charts: TT22

Name: TT22 [View in context] Type: Subroutine Category: Charts Summary: Show the Long-range Chart (red key f4)
.TT22 { LDA #64 \ Clear the top part of the screen, draw a white border, JSR TT66 \ and set the current view type in QQ11 to 32 (Long- \ range Chart) LDA #7 \ Move the text cursor to column 7 STA XC JSR TT81 \ Set the seeds in QQ15 to those of system 0 in the \ current galaxy (i.e. copy the seeds from QQ21 to QQ15) LDA #199 \ Print recursive token 39 ("GALACTIC CHART{galaxy JSR TT27 \ number right-aligned to width 3}") JSR NLIN \ Draw a horizontal line at pixel row 23 to box in the \ title and act as the top frame of the chart, and move \ the text cursor down one line LDA #152 \ Draw a screen-wide horizontal line at pixel row 152 JSR NLIN2 \ for the bottom edge of the chart, so the chart itself \ is 128 pixels high, starting on row 24 and ending on \ row 151 JSR TT14 \ Call TT14 to draw a circle with crosshairs at the \ current system's galactic coordinates LDX #0 \ We're now going to plot each of the galaxy's systems, \ so set up a counter in X for each system, starting at \ 0 and looping through to 255 .TT83 STX XSAV \ Store the counter in XSAV LDX QQ15+3 \ Fetch the w1_hi seed into X, which gives us the \ galactic x-coordinate of this system LDY QQ15+4 \ Fetch the w2_lo seed and clear all the bits apart TYA \ from bits 4 and 6, storing the result in ZZ to give a ORA #%01010000 \ random number out of 0, &10, &40 or &50 (but which STA ZZ \ will always be the same for this system). We use this \ value to determine the size of the point for this \ system on the chart by passing it as the distance \ argument to the PIXEL routine below LDA QQ15+1 \ Fetch the w0_hi seed into A, which gives us the \ galactic y-coordinate of this system LSR A \ We halve the y-coordinate because the galaxy in \ in Elite is rectangular rather than square, and is \ twice as wide (x-axis) as it is high (y-axis), so the \ chart is 256 pixels wide and 128 high CLC \ Add 24 to the halved y-coordinate and store in XX15+1 ADC #24 \ (as the top of the chart is on pixel row 24, just STA XX15+1 \ below the line we drew on row 23 above) JSR PIXEL \ Call PIXEL to draw a point at (X, A), with the size of \ the point dependent on the distance specified in ZZ \ (so a high value of ZZ will produce a 1-pixel point, \ a medium value will produce a 2-pixel dash, and a \ small value will produce a 4-pixel square) JSR TT20 \ We want to move on to the next system, so call TT20 \ to twist the three 16-bit seeds in QQ15 LDX XSAV \ Restore the loop counter from XSAV INX \ Increment the counter BNE TT83 \ If X > 0 then we haven't done all 256 systems yet, so \ loop back up to TT83 LDA QQ9 \ Set QQ19 to the selected system's x-coordinate STA QQ19 LDA QQ10 \ Set QQ19+1 to the selected system's y-coordinate, LSR A \ halved to fit it into the chart STA QQ19+1 LDA #4 \ Set QQ19+2 to size 4 for the crosshairs size STA QQ19+2 \ Fall through into TT15 to draw crosshairs of size 4 at \ the selected system's coordinates }