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eaten a penalty of 5 moves is incurred. Numerals 6, 7 or 8 are input as pseudo-cursor controls to move the insect down, up, or forwards respectively. The computer keeps track of the number of moves taken to reach column 32 and displays the total at the end of each game together with the best performance in the present series. Pressing NEWLINE after a game sets up another game in the same series. Entering a character starts a new series. POKEing 50 2 LET Y = 32000 No. of moves — best so 52 far! 4 INPUT R Seed for random number 54 generator 8 LET P = 0 56 10 LET W = 0 Assign variables prior to 58 PEEK 60 12 LET B = 1 and POKE 62 14 LET A = 8 64 16 LET Z = - 1 18 LET M = 0 66 20 RANDOMISE R Set seed for random number generator 68 22 FOR N = 1 TO 352 70 24 LET D « 9 72 26 LET X = RNDI2) 28 IF X = 1 THEN LET D = 128 Print eleven lines with 74 30 PRINT CHR$(D); black and grey squares at random. Pattern 500 determined by R. 510 32 NEXT N 520 34 GOSUB 500 Locate display-file 36 POKE W + 232, 20 Insect in initial position 530 AND FOR THE '81 While there are many similarities between the ZX80 and the ZX81 in the way the display is handled, there are some significant differences. 38 LET Z = Z + 1 40 INPUT C 42 GOSUB 500 44 LET M = W + (A — 1 >" 33 + B 46 POKE M, 0 48 IFC = 6 and A > 11 ORC = 7 AND A>1 THEN LET A = A —2*C+ 13 IFC = 8 THEN LET B = 8+1 LET M = W + (A - 1C33 + B IF PEEK(M) = 1 28 THEN LET Z = Z + 5 POKE M, 20 IF B = 32 THEN GOTO 62 GOTO 38 IF Z> Y THEN Y = Z PRINT "END OF GAME IN "Z," MOVES" PRINT "BEST SO FAR", "Y," MOVES" INPUT X$ CLS IF X$ = " "THEN GOTO 8 RUN LET P = PEEK(1 6397) P> 127 THEN LET P = P-256 LET W = PEEK(16396) 4 P* 256 RETURN Count No. of moves Which way? Locate display-file Th< adc CH dis| the Put a space where insect ;inc Set A and the make sure we don't at t POKE off-screen can line Mev Set M to next insect location address jse If there's a black square \7~fe the way, add penalty J)te Put insect in next locatior -j Watch for end of game jls Next move Set Y to best so far ^ he x>si >OK her i th< NEWLINE for another J?®', game Any character for anotha^OL series )3rc hrec Subroutine for setting W j to address of start of rt display-file . T s P (f P . T |ai C P< H
sves ile sre insect is ion*t n lsect s k square in snalty xt location jf game j far nother for another setting W tart of The display file is a block of addresses that contain the CHR$ codes that form the display on the TV screen. At the beginning of the file we find a Newline CHR$, and there are also Newline CHR$ at the end of each line. As we can print up to 32 CHR$ per line, every 33rd CHR$ = a Newline CHRS. These are used by the ZX81 ROM as references and should not be altered. To find the position of the display file we can use: 5 LET Z= 1 + PEEK 16396 + 256 * PEEK 16397. "Z" will now be the address of the first print position on the screen. If we POKE Z with any CHR$ code then this CHR$ will be printed n the first position on the position, and so on, as shown on the chart. To prevent printing, or should I say POKEing, to the 33rd CHR$ there are generally three different approaches. 1. To use a dedicated POKE routine that can never work out to hit a 33rd CHR$. 2. To peek at the position on the screen before POKEing, ie if PEEK (position) = 118 (Newline CHR$) then don't POKE here. 3. To use a formula that will automatically skip every 33rd CHR$, ie let us say X is the position we wish to poke then we can use POKE (Z + X + INT(X/32),(CHR$ code) this will automatically add one after every 32nd CHR$ making 33 = 34 effectively, and so on for every line. A simple program using this formula :- 5 LET Z= 1 + PEEK 16396 + 256*PEEK 16397 10 FOR X='TO 763 STEP 7 20 POKE (Z + X + INT(X/32)),8 30 NEXT X 40 STOP (Note:- That this formula automatically adds one to every 33rd position. The memory map chart must be read as (position) — (line number on left) to give the correct number per position). ANOTHER SHORT PROGRAM 5 LET Z = 1 + PEEK 16396 + 256*PEEK 16397 10 LET A = 0 20 LET B = 33 25 IF A< 0 OR A> 767 THEN GOTO 35 30 POKE Z + A + INT/A/32), 128 35 LET A = A + B 40 IF A>0 THEN LET B - 33 50 fF A>767 THEN LET B = -31 60 IF A = 27 THEN CIS -70 GOTO 25 POKEing ZX8t One thing to remember is that the display file is using the memory immediately after the BASIC program. This means that if the BASIC program is altered then "run" is required to find the new value for "Z", or at least the new value for "Z" must be calculated. An idea of a simple program may be as follows. We have a gun at the bottom left-hand side of the screen. Pressing any key from 0 to 9 will fire the gun at a target at the to line on the screen, "0" to fire straight up, "9" to fire farthest right, we will also need to PEEK at the top line in the relevant place to see if we - D H O l V f - PROGRAMMING have hit the target. Looking at this idea we can see that system 1 can be used as we don't intend to POKE over the edges of the screen, and the plotting involved has a limited path. After the normal line 5 to find "2" we then POKE Z + 726,1 28 to put a square block for the gun. Then to fire straight up to the top line is easy, we simply POKE Z + 693,23 then successively - 33 from this, poking all the way up to 0. Taking the top line to equal 32 CHR$ wide and the number of lines down to the gun being 22 we must shift to the right 32 + 22 = 1.5 positions per line to hit the top right-hand corner. Considering we have 9 shifted to the right options (0 to 9) if we use 1.5 + 9 = 0.166* (the INKEY$ number) this will give us the right amount of shift per line to plot our gun fire. I've rounded down to 0.16 so there is no chance of over-stepping the corner if 9 is selected. So the basic program is shown as, an INKEYS input, followed by the fire routine. (Note: - The line numbering should be followed to leave room for the rest of the program). UMMER 1983 z x COMPUTING SUMMER 1982 53
Page 1 and 2: j Summer Britain's Biggest Magazine
Page 3 and 4: ZX 80/81 HARDWARE/SOFTWARE ZX KEYBO
Page 5 and 6: ZX Computing is published quarterly
Page 7 and 8: 16K-byte RAM pock for massive add-o
Page 9 and 10: The @*?$1!&£@ RAM pack Dear ZX Com
Page 11 and 12: I u < o •0 8 & £T © Benchmarks
Page 14 and 15: 1 1© L" 20 30 IN5 5 0 60 80 9C 100
Page 16 and 17: The Explorer's Guide to the ZX81 IF
Page 18 and 19: Orbit S M King from Bristol takes y
Page 20 and 21: ID 15 20 25 30 35 4G> LBV 5 NMER RE
Page 22 and 23: E" OR Pi="M" THEN GOTO 785 CO)TO 7"
Page 25 and 26: I first became aware of the bug in
Page 27 and 28: A. Beasley After using the ZX80 for
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Page 31 and 32: i! I are odd jmeone to write a a ta
Page 33 and 34: id (and IC AND )MPUTER" er and >ut
Page 35 and 36: 16K Games Power Boat if you're desp
Page 37 and 38: }RTIO 3RT W J Hfl'.f K E COl T CORj
Page 39 and 40: 1220 \225 230 250 ,L NEED m 270 WAY
Page 41 and 42: IKING >S TO THi 350 >TCARD Ol OF 84
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Page 49 and 50: Micro This program it fascinating t
Page 51: 9 S e E k isplay-file we can POKEin
Page 55 and 56: — 32) limiting the firing to ten
Page 57 and 58: Winning Positions COIN COIN BELL BE
Page 59 and 60: REM "OTHELLO" St_GU GOTO 1103 LET 5
Page 61 and 62: lTURES P&P) ITAL iGUE write to IGHT
Page 63 and 64: ms, arid I time you convince somebo
Page 65 and 66: ist for the feel good luct their 5
Page 67 and 68: *t»m Tarshis, Executive Vice Presi
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Page 73 and 74: Review Keyboard Review Crofton's Ad
Page 75 and 76: "The hi-res board seems to go to a
Page 77 and 78: Make the most of IK Chopper Squad R
Page 79 and 80: onic Systems 80/1 ^ / RAMPACK MASSI
Page 81 and 82: 51 IF A(2) «* 188 THEN LET A (Z)
Page 83 and 84: 4110 IF (A(Z - 13) = 1 OR A(2 - 13)
Page 85 and 86: ZX81 HARDWARE 40 KEY KEYBOARD. Kit
Page 87 and 88: 4090 GOTO 9100 5000 GOSUB 9500 5010
Page 89 and 90: I 1K Games 10 LET 20 LET 30 LET 4-0
Page 91 and 92: Etchasketch This very short program
Page 93 and 94: meant the addition to its repertoir
Page 95 and 96: SYSTEM/MULTIFUNCTION COMMANDS BREAK
Page 97 and 98: 270 280 290 300 310 320 IF IF IF IF
Page 99 and 100: Make sure you get every issue of ZX
Page 101 and 102: ZX80 Programs ZX80 fights back Musi
Page 103 and 104:
Catch Avoid being trapped by your s
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Software Orwin strikes again Casset
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ZX81 Software LARGE CHARACTERS REM
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Moving graphics Getting into the Mo
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Specialist Book§ i Choosing progra
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WHAT CAN I DO WITH 1K? If your answ
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Compiler A program whose function i
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ZX COMPUTING SUMMER 1982 Computer G
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S-100 Name of a bus or connection s
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After taking your move the computer
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WHAT'S ON NEXT? IN HOBBY ELECTRONIC
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are not lost when the ZX81 is turne
Page 131 and 132:
ZX Users' Club JOIN YOUR USERS' GRO
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