Patterns

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Mastering Machine Code on Your Spectrum


part 5 of 8 - from ZX Computing Jun/Jul'83





Machine code master, Toni Baker, shows you how to


incorporate machine code within your BASIC programs with an


incredible program to add visual accompaniment to your


stereo.








I'd like to cheat a bit now, if I may, by giving you a BASIC


program. (Shrieks of horror!) Well, it has got some machine


code in it, but nothing new. This is a lesson in how to


incorporate machine code into BASIC, for finding a use for


all these weird and wonderful routines that keep cropping


up. Almost the hardest task you have in programming is the


hanging around at bus stops and tedious dinner parties


waiting for that most illusive of qualities - inspiration.





The machine code routine in question was featured in part


three of this series of articles - it's a routine to change


the colours of PAPER and INK throughout the whole screen


faster than you can say 'The sixth sick sheik's sixth


sheep's sick' without falling over. The BASIC that surrounds


it is new, however. This is a program to impress your next


door neighbours and fanatics of laser shows, or a new way of


running a disco. This is visual accompaniment to your


stereo!








Sound's great!





It's not technically a sound- to- light unit, for the


program has no way of knowing whether or not your stereo is


even switched on, let alone what's playing on it. What it


is, however, is a very good optical illusion of sound and


vision being synchronised. What you do is RUN the program


and input answers to the various questions asked, put your


favourite record on, switch the lights off and close the


curtains, then just sit back while your brain dances round


in circles.





The program is featured in Figs. 1 and 2. You should enter


the machine code first (using a BASIC program to do so) and


then delete this BASIC program to input the one given. Take


a look at this now.





I'll now give you some examples on how to RUN the program.


Type RUN. (Easy so far, isn't it?) For the question 'NUMBER


OF LINES', you should input (say) 5. For 'INK', input 'INT


(4*RND)+4', and for 'PAPER' input '0'. Finally, for 'STARS?'


you should input 'Y'. This is just an example - try it out


for yourself.








What's the plot?





The program has a couple of extra features which you ought


to know about. RUN 200 enables you to define the initial INK


and the initial PAPER colours. For instance, RUN 200 and


then input 0 / 7 / 4 / '7-X' / '7-Y' / 'Y' / ('/' counts as


ENTER). RUN 400 will SAVE and VERIFY the program and the


machine code.





I won't turn the above program into machine code just yet.


Before I do, I'll give you some information on PLOT and


DRAW.





CALL PLOT_BC (Hex CD E522) requires B to hold the Y


co-ordinate, and C to hold the X co-ordinate. This will PLOT


the required point. Bit zero of P_FLAG (5C91) must be zero


for OVER 0 or one for OVER 1.





CALL DRAW_3 (Hex CDBA24) requires B to hold the absolute


value of the Y parameter, and C the absolute value of the X


parameter. If Y is greater than or equal to zero then D


should hold 01 Hex, otherwise D should hold FF Hex. If X is


greater than or equal to zero then E should hold 01 Hex,


otherwise E should hold FF Hex. The sequence of instructions


LD HL,2758 / EXX must be carried out between CALL DRAW_3 and


RET (to BASIC) otherwise the Spectrum will crash.








Transformation time





We still can't rewrite the program into machine code yet,


since we don't have a routine for INT (X*RND). It is


possible to write a simple random number subroutine which


creates random numbers between 0000 and FFFF Hex in the HL


register pair, so take a look at Fig.3 which illustrates


such a subroutine. It works using the random number seed


already used by the ROM, but does not actually call the


ROM's RND routine (since this is highly impractical). The


subroutine does, however, work in more or less the same way


as RND - it takes the value of the system variable SEED, and


multiplies it by some constant. Only the remainder modulo


65536d is taken as the new result. This is not quite RND,


but it does give fairly random results.





Figure Four, on the other hand, is another kettle of fish


altogether. This is a subroutine I've called RAND_A in order


to distinguish it from the subroutine in Fig.3. It requires


that A contains a number between 00 and A-1. Of particular


note is the subroutine MULT which is called from within


RAND__A; this is a subroutine which will multiply the number


held in the A register by the number held in the DE register


pair. The result will be formed in the AHL register triplet


(the result will always fit in three bytes). RAND_A works


just like the ROM's RND routine - it takes a value, SEED,


which is multiplied by 4B, and then the remainder is found


from a division by Hex 10001, less one. Can you see how the


program calculates this remainder? To find the random number


required, this new seed is multiplied by A and the high part


only becomes the random number.





Now we are almost ready to turn the program Patterns


entirely into machine code. Well - not quite entirely as we


still can't do VAL. Let's ignore that part for the time


being and leave VAL in BASIC. I'd like you now to study


Figs. 5 and 6, which list the revised program, now called


Patterns 2. If you now compare it with Figs. 1 and 2, see if


you can work out how the translation is achieved. Note that


the instruction CALL RAND_A is used - this is a reference to


the subroutine in Fig. 4.





You can use any addresses you like for this program. I chose


to use the following:





Label Hex Dec


RAND_A 7000 28672


MULT 7021 28705


FIND_A 7030 28720


PL_DR 703B 28731


INIT 7078 28792


MAIN 7098 28824


P_VAL 7105 28933


P_OVER 7106 28934


P_DRAW 7107 28935


LINES 7108 28936


INK 7109 28937


PAPER 710A 28938


STARS 710B 28939


ARRAY 710C 28940





It is essential, however, that P_OVER and P_DRAW be


adjacent, and also that ARRAY points to the start of a


segment of spare RAM.








- Fig.1 The program, Patterns: part one - the BASIC - - - - - - - - - - - -





10 INPUT "NUMBER OF LINES";n


20 DIM x(n+1): DIM y(n+1)


30 INPUT "INK";x$ (use the keyword INK here)


40 INPUT "PAPER";y$ (use the keyword PAPER here)


50 INPUT "STARS?";a$: LET s=a$="N" OR a$="n"


60 CLS: LET p=1


70 LET p1=p+1: IF p1>n+1 THEN LET p1=1


80 LET p2=p1+1: IF p2>n+1 THEN LET p2=1


90 PLOT x(p1),y(p1)


95 DRAW x(p2)-x(p1),y(p2)-y(p1)


100 PLOT OVER s;x(p1),y(p1)


105 DRAW OVER 1;x(p2)-x(p1),y(p2)-y(p1)


110 LET x(p1)=INT (256*RND)


120 LET y(p1)=INT (176*RND)


130 PLOT x(p),y(p)


140 DRAW x(p1)-x(p),y(p1)-y(p)


150 LET x=VAL x$: LET y=VAL y$


160 POKE 32769,x: POKE 32770,y: LET p=p1+USR 32768


170 GO TO 70


200 INPUT "INITIAL INK";x


210 INPUT "INITIAL PAPER";y


220 GO TO 10


400 SAVE "PATTERNS" LINE 430


410 SAVE "PATTERNS" CODE 32768,34


420 VERIFY "": VERIFY "" CODE: STOP


430 CLEAR 32767: LOAD "" CODE: STOP [I added the CLEAR. JG.]








- Fig.2 The program, Patterns: part two - the machine code - - - - - - - -





01???? PATTERNS LD BC,???? To be POKEd by BASIC


78 LD A,B


76 HALT


D3FE OUT (FE),A


78 LD A,B


87 ADD A,A


87 ADD A,A


87 ADD A,A


81 ADD A,C


218D5C LD HL,ATTR_P


77 LD (HL),A


210058 LD HL,ATTRS


110158 LD DE,ATTRS+1


01FF02 LD BC,02FF


77 LD (HL),A


EDB0 LDIR


C9 RET








- Fig.3 The machine code random number subroutine - - - - - - - - - - - - -





D5 RAND PUSH DE


2A765C LD HL,(SEED)


54 LD D,H


5D LD E,L


29 ADD HL,HL


29 ADD HL,HL


19 ADD HL,DE


29 ADD HL,HL


29 ADD HL,HL


29 ADD HL,HL


19 ADD HL,DE


22765C LD (SEED),HL


D1 POP DE


C9 RET








- Fig.4 The INT(A*RND) routine - - - - - - - - - - - - - - - - - - - - - -





C5 RAND_A PUSH BC


D5 PUSH DE


E5 PUSH HL


F5 PUSH AF


3E4B LD A,4B


ED5B765C LD DE,(SEED)


CD???? CALL MULT


A7 AND A


4F LD C,A


ED42 SBC HL,BC


3801 JR C,AA


2B DEC HL


22765C AA LD (SEED),HL


54 LD D,H


5D LD E,L


F1 POP AF


CD???? CALL MULT


E1 POP HL


D1 POP DE


C1 POP BC


C9 RET


0608 MULT LD B,08


210000 LD HL,0000


29 LOOP ADD HL,HL


17 RLA


3003 JR NC,BB


19 ADD HL,DE


CE00 ADC A,00


10F7 BB DJNZ LOOP


C9 RET








- Fig.5 The program, Patterns2; the BASIC - - - - - - - - - - - - - - - - -





10 INPUT "NUMBER OF LINES";n


20 POKE lines,n: LET n=USR init


30 INPUT "INK";x$


40 INPUT "PAPER";y$


50 INPUT "STARS?";a$: POKE stars,a$="N" OR a$="n"


60 POKE ink,VAL x$: POKE paper,VAL y$


70 LET n=USR main


80 GO TO 60


200 INPUT "INITIAL INK";x


210 INPUT "INITIAL PAPER";y


220 GO TO 10


400 SAVE "P2" LINE 450


410 SAVE "P2" CODE rand_a,263


420 VERIFY ""


430 VERIFY "" CODE


440 STOP


450 CLEAR rand_a-1: LOAD "" CODE [I added the CLEAR. JG.]








- Fig.6 The program, Patterns2; the machine code - - - - - - - - - - - - -





D5 FIND_A PUSH DE


11???? LD DE,ARRAY


6F LD L,A


2600 LD H,#00


29 ADD HL,HL


19 ADD HL,DE


D1 POP DE


C9 RET


C5 PL_DR PUSH BC


D5 PUSH DE


7A LD A,D


D5 PUSH DE


CD???? CALL FIND_A


4E LD C,(HL)


23 INC HL


46 LD B,(HL)


C5 PUSH BC


3A???? LD A,(P_OVER)


32915C LD (P_FLAG),A


CDE522 CALL PLOT_BC


C1 POP BC


D1 POP DE


7B LD A,E


CD???? CALL FIND_A


7E LD A,(HL)


23 INC HL


1E01 LD E,#01


91 SUB C


3004 JR NC,PD2


ED44 NEG


1EFF LD E,#FF


4F PD2 LD C,A


7E LD A,(HL)


1601 LD D,#01


90 SUB B


3004 JR NC,PD3


ED44 NEG


16FF LD D,#FF


47 PD3 LD B,A


3A???? LD A,(P_DRAW)


32915C LD (P_FLAG),A


CDBA24 CALL DRAW_3


D1 POP DE


C1 POP BC


C9 RET


210040 INIT LD HL,D_FILE


110140 LD DE,D_FILE+1


01FF17 LD BC,#17FF


3600 LD (HL),#00


EDB0 LDIR


ED4B???? LD BC,(LINES-1)


04 INC B


21???? LD HL,ARRAY


AF XOR A


77 IN2 LD (HL),A


23 INC HL


77 LD (HL),A


23 INC HL


10FA DJNZ IN2


32???? LD (P_VAL),A


C9 RET


ED4B???? MAIN LD BC,(P_VAL)


3A???? LD A,(LINES)


51 LD D,C


14 INC D


BA CP D


3002 JR NC,M2


1600 LD D,#00


5A M2 LD E,D


1C INC E


BB CP E


3002 JR NC,M3


1E00 LD E,#00


210000 M3 LD HL,#0000


22???? LD (P_OVER),HL


CD???? CALL PL_DR


3A???? LD A,(STARS)


32???? LD (P_OVER),A


3E01 LD A,#01


32???? LD (P_DRAW),A


CD???? CALL PL_DR


7A LD A,D


CD???? CALL FIND_A


3EFF LD A,#FF


CD???? CALL RAND_A


77 LD (HL),A


23 INC HL


3EB0 LD A,#B0


CD???? CALL RAND_A


77 LD (HL),A


5A LD E,D


51 LD D,C


210000 LD HL,#0000


22???? LD (P_OVER),HL


CD???? CALL PL_DR


7B LD A,E


32???? LD (P_VAL),A


76 HALT


3A???? LD A,(PAPER)


D3FE OUT (#FE),A


87 ADD A,A


87 ADD A,A


87 ADD A,A


2A???? LD HL,(INK)


B5 OR L


328F5C LD (ATTR_T),A


210058 LD HL,ATTRS


110158 LD DE,ATTRS+1


01FF02 LD BC,#02FF


77 LD (HL),A


EDB0 LDIR


215827 LD HL,#2758


D9 EXX


C9 RET








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Another Fine Product transcribed by:


Jim Grimwood (jimg@globalnet.co.uk), Weardale, England


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