Re: How did Treasure Island work?

From	Volker Bartheld <news2020@bartheld.net>
Newsgroups	comp.sys.sinclair
Subject	Re: How did Treasure Island work?
Date	2020-07-16 12:30 +0200
Organization	bartheld.net
Message-ID	<csj6yfu76aqj.dlg@news.bartheld.net> (permalink)
References	<a157acca-9792-4459-9e87-b28033afb0f7n@googlegroups.com>

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On Thu, 16 Jul 2020 00:29:23 -0700 (PDT), Groo Vee wrote:
> There was a game, I'm _99%_ certain it was called Treasure Island, which
> had the top half of the screen blue and the bottom black (or maybe it
> was the other way around) INCUDING THE BORDER!! ie. the ENTIRE SCREEN'S
> TOP HALF was blue!!! This continued WHILE YOU PLAYED THE GAME!!!!!! It
> blew my mind at the time, so today, so many years later, I'd love to
> know how the HELL they pulled that off? What's going on?

Maskable interrupt handler [1], typically of the IM2 type (vectored
interrupt) where - as opposed to the first example in [1] - the complete
table is filled with #FEFE, so your interrupt handler always starts at
address #FEFE, no matter what lower byte is provided by the "peripheral
device" on the bus (since there usually is none or some Kempston joystick
interface producing garbage).

At #FEFE you would then have

	ORG #FEFE
	; no need to di, Z80 does this automatically
	push af ; save carry flag
	push bc ; save bc register

set border to blue

	ld a,#01	; blue border
     	out (#FE),a	; change color
	
and wait a multiple of 48+176=224 T-States [2]

	ld bc,6408h
wait:	nop		; 4T
	nop		; 4T
	nop		; 4T
	dec bc		; 6T
	jp nc, wait	; 10T
	; 28T for each loop, so 8 iterations gets you down one scan line
	
then change the border color to black

	ld a,00h	; black border
     	out (FEh),a	; change color

, restore the registers and return from the interrupt handler

	pop bc		; restore bc
	pop af		; restore af
	ei		; re-enable interrupts
	reti		; return

Hint: Don't ever use HALT in an interrupt handler and stay away from
contended memory (everything below #8000 is managed by the ULA). Have a
look at R. Zak's book "Programming the Z80" for details on machine code
timing [3].
	
Of course the example above is far from perfect (needs some nudging back
and forth to perfectly syncronize with the start of a scanline, you could
use OUTI/OTIR for even crazier stuff) and you can do this not only to the
border, but also the attribute area (using LDI/LDIR), getting you
multicolored attributes that overcome the 8x8 pixel limit.

This is what I wrote, decades ago. It even returns you to ZX BASIC with the
highres attributes on.

10 CLEAR 65369
20 LET d=65370
30 FOR a=1 TO 33
40 LET c=0
50 READ a$
60 READ check
70 FOR e=1 TO 10 STEP 2
80 LET b=(CODE a$(e)-48-(7 AND CODE a$(e)>57))*16
90 LET b=b+CODE a$(e+1)-48-(7 AND CODE a$(e+1)>57)
100 PRINT AT 10,0;d,b,: POKE d,b: LET d=d+1: LET c=c+b: NEXT e
110 IF d>65535 THEN GO TO 160
120 IF c<>check THEN PRINT "CHECKSUMERROR IN LINE ";(a-1)*10+9000: LET f=(a-1)*10+9000: LIST f: STOP 
130 NEXT a
140 CLS : PRINT "HIGHRESOLUTION COLOUR ATTRIBUTES"'''
150 PRINT "START:",65370'"LENGTH:",166'"INTERRUPT ON:",65370'"INTERRUPT OFF:",65384'''"LABELS:"'"STARTLN:",65393;" L=1"'"DEPTH:",65394;" L=1"'"HIATT:",65395;" L=2"'"IHANDLE:",65524
160 STOP 
170 SAVE *"m";1;"HIGHRES-I"CODE 65370,166: VERIFY *"m";1;"HIGHRES-I"CODE 
180 STOP 
9000 DATA "F321FFFF36",840
9010 DATA "183E3BED47",453
9020 DATA "ED5EFBC9F3",1026
9030 DATA "3E3FED47ED",670
9040 DATA "56FBC90018",562
9050 DATA "0000000000",0
9060 DATA "00C5D5E5F5",884
9070 DATA "ED7375FF31",773
9080 DATA "79FF011802",403
9090 DATA "0B78B120FB",591
9100 DATA "3A71FF8787",696
9110 DATA "87CAA0FF06",758
9120 DATA "0F10FE0000",285
9130 DATA "C83DC295FF",859
9140 DATA "6F3A71FF67",640
9150 DATA "CB3CCB1DCB",698
9160 DATA "3CCB1DCB3C",555
9170 DATA "CB1D110C58",349
9180 DATA "19EB2A73FF",672
9190 DATA "3A72FF0140",492
9200 DATA "00D5EDA0ED",847
9210 DATA "A0EDA0EDA0",954
9220 DATA "EDA0EDA0ED",1031
9230 DATA "A0EDA0D1E2",992
9240 DATA "DBFF180000",498
9250 DATA "E6FF18E4EB",972
9260 DATA "0E2009EB3D",351
9270 DATA "C2BCFFED7B",997
9280 DATA "75FFF1E1D1",1047
9290 DATA "C1FBC33800",695
9300 DATA "00000000F3",243
9310 DATA "C379FF0000",571
9320 DATA "0000000000",0
9330 DATA "0000000000",0
9340 DATA "0000000000",0

which disassembles as (no warrantee, no comments):

        ORG     0FF5Ah

        DI
        LD      HL,0FFFFh
        LD      (HL),18h
        LD      A,3Bh
        LD      I,A
        IM      2
        EI
        RET

LFF68:  DI
        LD      A,3Fh
        LD      I,A
        IM      1
        EI
        RET

LFF71:  NOP
LFF72:  JR      LFF74

LFF74:  NOP
LFF75:  NOP
        NOP
        NOP
        NOP
LFF79:  PUSH    BC
        PUSH    DE
        PUSH    HL
        PUSH    AF
        LD      (LFF75),SP
        LD      SP,0FF79h
        LD      BC,0218h
LFF87:  DEC     BC
        LD      A,B
        OR      C
        JR      NZ,LFF87
        LD      A,(LFF71)
        ADD     A,A
        ADD     A,A
        ADD     A,A
        JP      Z,LFFA0
LFF95:  LD      B,0Fh
LFF97:  DJNZ    LFF97
        NOP
        NOP
        RET     Z
        DEC     A
        JP      NZ,LFF95
LFFA0:  LD      L,A
        LD      A,(LFF71)
        LD      H,A
        SRL     H
        RR      L
        SRL     H
        RR      L
        SRL     H
        RR      L
        LD      DE,580Ch
        ADD     HL,DE
        EX      DE,HL
        LD      HL,(LFF72+1)
        LD      A,(LFF72)
LFFBC:  LD      BC,0040h
LFFBF:  PUSH    DE
        LDI
        LDI
        LDI
        LDI
        LDI
        LDI
        LDI
        LDI
        POP     DE
        JP      PO,LFFDB
        JR      LFFD6

LFFD6:  NOP
        AND     0FFh
        JR      LFFBF

LFFDB:  EX      DE,HL
        LD      C,20h
        ADD     HL,BC
        EX      DE,HL
        DEC     A
        JP      NZ,LFFBC
        LD      SP,(LFF75)
        POP     AF
        POP     HL
        POP     DE
        POP     BC
        EI
        JP      0038h

LFFF0:  NOP
        NOP
        NOP
        NOP
        DI
        JP      LFF79

Also,

https://www.youtube.com/watch?v=eGDLdf3nZL0
https://www.youtube.com/watch?v=wE1KylKQDrQ
https://www.youtube.com/watch?v=79sg7UE6KAA
https://www.youtube.com/watch?v=VB3mDqGgBag

should give you a good impression of what is possible.

HTH,
Volker

[1] https://codersbucket.blogspot.com/2015/04/interrupts-on-zx-spectrum-what-are.html
[2] http://www.zxdesign.info/interrupts.shtml
[3] http://www.z80.info/zaks.html

Thread

How did Treasure Island work? Groo Vee <groovee@cyberdude.com> - 2020-07-16 00:29 -0700
  Re: How did Treasure Island work? Volker Bartheld <news2020@bartheld.net> - 2020-07-16 12:30 +0200
    Re: How did Treasure Island work? Chupo <bad_n_mad@yahoo.com> - 2020-08-27 05:08 +0200
      Re: How did Treasure Island work? Volker Bartheld <news2020@bartheld.net> - 2020-08-27 08:02 +0200

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