A VARIABLE DISPLAY ROUTINE

WITH MULTITASKING FEATURES

Revised 19/04/2005

INTRODUCTION

NOVA1000 is a 512 byte machine code routine which replaces the SINCLAIR video

driver and provides some powerful new display facilities. These include:

1. Increased horizontal line length with up to 34 characters per line.

2. Increased number of lines with up to 29 lines (in 50Hz mode)

3. No N/L character (CHR$ 118) required to terminate video lines.

4. Sliding display window on memory permits horizontal and vertical scrolling.

5. Rapid switching of Display files facilitates screen animations.

6. Increased execution speed with smaller screen displays for fast action.

7. View contents of large A$ string array directly

In addition to these display features, NOVA1000 includes several new system

utilities such as

1. Repeating keys.

2. 100 Hour clock/timer

3. BASIC line number trace.

A USER JUMPVECTOR allows advanced users to add more short m/c routines or

develop a multitasking system to run several longer programs concurrently.

NOVA is designed to run above RAMTOP. When loaded from tape the NOVA machine

code program is in a 0 REM line and RAND USR 16535 lowers RAMTOP and then

relocates NOVA. The accompanying BASIC program provides an example of using

the various NOVA screen modes and other features. The user program would

normally be combined with the 0 REM line and replace the NOVA BASIC demo.

A copy of the demo program NOVA2005 is found here NOVA2005.p

NOVA DISPLAY - POSITION AND SIZE

The most powerful feature of this program is the ability to create new Video

Display files located anywhere in memory. A variable number of character per

line, a variable number of lines per screen and the starting position (memory

address) of the screen are under program control.

The NOVA video routines create a window on the memory and this window can be

rapidly moved from one location to another by changing the OFFSET parameter.

The height of this window is controlled by the LINES parameter 1 to 28 lines.

Above the window is the STATUS line which displays the TIMER/CLOCK (T/C)

and the TRACE line number.

Below the window the 24th line from D-FILE may be optionally displayed. This

is the BASIC COMMAND/EDIT/INPUT line.

Like the normal screen, certain illegal character codes must be avoided.

These illegal character codes include CHR$64 to 127 and 192 to 256 with

exception of CHR$ 118 (N/L).

The ZX D-FILE display can be viewed without problems and the A$ String

Variable can be used as one large screen or multiple smaller Vfile screens.

The ZX D-FILE can be 1 to 23 lines high and 32 characters side with the 24^th

D-FILE edit line optionally shown.

To view a normal D-FILE screen for BASIC listings etc, the screen location

parameter OFFSET is set to 0000. Since the normal expanded display file

contains N/L characters, the line length is 32 characters. The height of the

screen should be set for 23 and the COMMAND line turned on, to view the normal

24^th edit line. For faster program execution turn off line 24 and or use fewer

displayed D-FILE lines. Maximum program speed is achieved with 1 displayed line.

The A$ VFILE can be much bigger than the screen display and the display can

be moved like a window over this string array. All normal string functions

can then directly operate on the screen. The Vfile does not require N/L characters

to delimit each displayed line and as a result the lines are 34 characters wide.

In order to view the Variable Array, the screen location parameter OFFST is set

to 1 or more determining the position of the start of the Vfile window with

respect to the start of the variable array, but how do we ensure that no portion

of memory after the end of the array displayed?

This problem of accidentally displaying data beyond the Vfile display is

overcome by the use the Screen OFfseT (SOFT) routine, which tests the length

of the array with the OFFSET value and sets this value to 0000 if the offset

is too large and would result in display of data beyond the end of the array.

In the direct edit mode or as part of a program, the SOFT routine is called

in the form RAND USR 32680 + N where N is the offset.

Horizontal scrolling is achieved by changing OFFSET by +/- 1, vertical

scrolling by changing OFFSET by +/- 34 and screens are paged by changing

OFFSET +/-(34 x LINES)

If the OFFSET parameter is poked DIRECTLY to point to any other part of

memory (ie above 32k) then the user has to ensure that this memory to be

viewed is free of illegal character codes.

The parameter LINES holds the number of lines between the top status line

and the bottom command line in the range of 1 to 26.

The normal screen or D—FILE has only 24 lines of characters that can be

viewed and any attempt to poke more into LINES may result in a system crash.

When viewing less than 22 lines, the SLOW MODE execution of BASIC and M/C

speed up inversely proportional to the number of viewable lines on the screen.

The maximum speed is attained when the value of LINES is 1 and TRACE is off

Finally the number of characters per line can be varied by inserting a N/L

character in the line. However with N/L’s scrolling is more difficult, for

example the vertical scroll must be controlled by adjusting the OFFSET with

respect to the next N/L character or 34 characters whichever is less.

TIMER/CLOCK

The timer routine operates directly on 8 digits plus 3 : delimiters in the

STATUS line. The routine increments the least significant digits (1/60 sec per

tick) once per frame. At 59 ticks the LSD change to zero and increments the

Second digits, which at 59+1 seconds increments Minutes, which at 59+1 minutes

increments Hours up to 99 hours. A 12 or 24 hr clock can be implemented with

a short BASIC routine which resets the hours at 13hrs to 1hr or at 24hrs to

0 hr respectively.

The timer and in fact the entire status line can be transferred to the COMVAR

variable as a 32 character string with RAND USER 32677. Conversely the

COMVAR contents can be transferred to the status line with RAND USR 32674

COMVAR

The Communication Variable (COMVAR)is a string variable with a user defined name.

COMVAR is initialized with DIM S$(RAND USR 32671) to 32 characters and a copy of

the name, saved in the ATFR transfer byte is used to later find the variable.

The first 11 characters correspond to the TIMER/CLOCK digits and this is how the

TIMER is set up:

LET S$(1 TO 12) = ”00:00:00:00”

RAND USR 32674

To read the timer and compare against alarms, etc. you can use a string function

IF S$(USR 32761 + 1 TO 11) > “10:11:12:13” THEN GOTO ALARM1

THE NOVA FLAG

The first 3 bits in the FLAG byte at location 32625 can be poked to start and

stop various NOVA features.

The TRACE of BASIC lines numbers is a nice debug feature, which can tell you

where the BASIC program spends most of its time. FLAG Bit 0 = 1 turns on the

TRACE feature.

Repeat key causes multiple key entries when held down. This is not exactly a

typomatic routine so the speed depends on the calling program and number of

lines displayed. Still I have found it useful when a key must be recognized.

REPEAT applies to the entire keyboard. FLAG bit 1 = 1 turns on REPEAT keys.

In some cases, it is desirable to control the visibility of the bottom command

line. For example, turning off the command line speeds up program execution.

FLAG bit 2 = 1 turns the COMMAND LINE on.

These are the values that may be poked into 32625

(0) trace off, repeat off, command line off

(1) trace on , repeat off, command line off

(2) trace off, repeat on , command line off

(3) trace on , repeat on , command line off

(4) trace off, repeat off, command line on

(5) and so on to 7

The Line byte is at 32626. This byte controls the number of display

lines and therefore the speed of user BASIC or M/C programs.

It is up to five times faster than the SLOW mode and 80% as fast as the

normal video FAST mode if the number of lines is reduced to 1 and

FLAG is set to zero.

EXIT TO ZX BASIC

Exit to normal ZX video by using FAST, SLOW, SAVE, LOAD or PAUSE xxx in

a BASIC program line. When in the EDIT mode while NOVA is running EXIT to

ZX video by entering a dummy program line (e.g. 1 , N/L)

Note: It is important to remember that the NOVA timer/clock timer does not

run in when returning to ZX Video.

The user vector is a three byte address starting at 32631. Location 32631 is

initially set to C9 (RET) but can be set to JP xxxx a short user routine

which must end with RET. If the JP instruction is poked always start with the

address (32632 – LSB, 32633 – MSB before poking the C3 JP code into 32631.

MULTITASKING MACHINE CODE PROGRAMS

For advanced programmers NOVA can be revised and extended with additional

Machine Code routines. The NOVA video routine is called 120 times per second

by the NMI service routine and this provides a means to insert user routines

which run concurrently with NOVA and any other basic or M/L programs.

The user may provide 2 types of M/C routines to run concurrently with NOVA

and BASIC.

The first type is a SHORT M/C ROUTINE and it is executed as part of the

program but must RETURN before NOVA itself returns to the BASIC (or MC)

program.

The second type is the LONG M/C PROGRAM and this program is alternating with

the main program 60 times per second, using up 50% of the available CPU time.

SHORT USER M/C ROUTINE

The jump vector at 32631 is provided so that NOVA may be expanded to include

user supplied routines. This may take the form of a simple and short program,

which is terminated with a RET instruction. It must execute in the time

between the end of the vertical sync routine and the beginning of the active

display.

During this time, the trace routine is also executed if enabled.

Failure to observe the time limitation may return the system to the normal

screen or sometimes cause a system crash. As pointed out, some more time is

available if the TRACE is never on at the same time as the user routine.

It is recommended that the FLAG bit 4 is tested to make a conditional

RETURN Z if FLAG bit 4 is not enabled and in this way the user and TRACE

Routine can be simultaneously controlled using POKE 32625,n or LD ($7F71),n

What can we do with such a short routine?

1. PRINTER SPOOLER - the contents of a text file may send to the printer

while

the current foreground program is executing.

2. Sensors may be scanned as part of a background home control or alarm

program

3. Voice synthesizer may be loaded with phoneme data while displaying

characters on the screen.

4. Joystick port can be read and characters entered as if keys were pressed.

5. TIMER/CLOCK and REPEAT KEY are examples of the SHORT M/C ROUTINE.

Despite the limits of available time, a SHORT M/C ROUTINE can do a lot.

LONG USER M/C ROUTINE

The time limitation of the short user routine may be overcome with a special

routine call TASKSWAP, which is itself a short user routine but controls two

or more long programs (tasks). One drawback is the need for separate M/L

stacks to save the register contents of each task waiting to be executed. The

size of the stacks depends on the number of nested CALLS and recursions.

In this version of TASKSWAP the user must reserve the stack space (usually

256 bytes) and pass the start location of the tasks to the task file.

The user can now install a second main program (must be M/L program), which

runs concurrently with the other main program (M/L or Basic) ,NOVA, and any

other short routines. This means that we can run a M/L program which passes

some jobs to a second program ( floating point calculations) but continues

with occasional checks to see if the results are available.

The set up for running alternating tasks is not a trivial problem and

presumes that the user has some knowledge of the operating system as well as

the means to generate his or her own programs.

For serious investigation of ZX multitasking, users should develop their own

utility programs to make this job easier.

A suggested first utility is TASKEDITOR, to start and stop tasks controlled

by the Timer/Clock and to pass data between tasks.

Also required is a WINDOWMANAGER, which transfers data from individual tasks

to the video window associated with that task The natural approach is to

define various starting locations within the viewable array as the start of

windows assign to each task and swap them to the screen as needed.

Details of this are discussed in the section on display size and position.

A refinement would be multiple layered windows on one screen each with it’s

own priority, size and position which show the viewable data produced by

each task

These and other MULTITASKING UTILITIES have to wait for a future installment.

logging off: WILF R.

POKES USR

POKE 32625,0 TO 15-FLAGS RAND USR 32668 -START NOVA

POKE 32626,1 TO 26-LINES DIM Z$ (USR 32671) -DEFINE COMVAR (B$-Z$)

POKE 32627/28,0 TO 64K-OFFSET RAND USR 32674 -MOVE COMVAR TO STATUS

POKE 32631/3,JP VECTOR (RET) RAND USR 32677 -MOVE STATUS TO COMVAR

POKE 32634/64,”XX..X”-STATUS RAND USR 32680+N -USER FRIENDLY OFFSET

;NOVA SOURCE CODE LISTING

;COMPATIBLE WITH ARTIC ASSEMBLER

;** NEW VIDEO VECTOR **

STRT1 LD IX,VDR ;NEVER CALL VDR DIRECTLY

RET

;** MAIN LOOP **

VDR CALL SDP ;STATUS LINE (RTC)

CALL FRM ;RTC UPDATE

CALL VDP ;VARIABLE LENGTH DFILE OR A$

;CALC BOTTOM BLANK LINES

CALL ZDP ;ZX COMMAND LINE 24 (?)

CALL RTN ;KEY REPEAT(?) AND RETURN TO BASIC

VDR1 CALL SNC ;FRAMES, KEYBOARD, VERTICAL SYNC

LD B,07 ;ALIGN SCREEN

CALL DLY ;VARIABLE DELAY ROUTINE

LD A,EC ;NUMBER OF TOP BLANK LINES

EX AF ;SAVE FOR NMI

CALL TRCE ;BASIC LINE TRACE ROUTINE

CALL USR ;DO USER ROUTINE

LD IX,VDR ;RESTORE VDR JUMP VECTOR

JP 02A4 ;EXIT TO BASIC UNTIL NEXT VDR

;** VARIABLE DELAY ROUTINE **

DLY NOP ;DELAY TIME DEPENDS ON

DLY0 NOP ;REGISTER B AND ENTRY POINT

DLY2 NOP ;

DLY3 NOP ;

DLY1 NOP ;EACH NOP = 4T STATES

DLY4 NOP ;PLUS REG B X 12T + 7T

DLY5 NOP

DLYB DJNZ DLYB

RET

;** VSYNC VIA KEY SCAN **

SNC PUSH AF ;DUMMY REGISTER PUSH TO

PUSH BC ;FORCE RETURN TO VDR (NOT BASIC)

PUSH DE

PUSH HL

JP 0229 ;FRAMES, KEYBOARD AND VSYNC

RTN LD A,(FLAG) ;CHECK FLAG

BIT 1,A ;FOR REPEAT BIT 1 SET

JR Z RTN1 ;EXIT IS NO REPEAT

XOR A ;REPEAT SET SO

LD (4027),A ;RESET DEBOUNCE SYSVAR

RTN1 POP IX ;SAVE RETURN ADDRESS IN REG IX

JP 02A2 ;START NMI AND RETURN TO BASIC

;** TOP OF DISPLAY SCREEM **

SDP LD B,02 ;SYNCHRONIZE WITH DISPLAY

CALL DLY ;

LD HL,RTCL ;POINT TO THE STATUS LINE

JR ZDP1 ;AND DISPLAY IT

;** MIDDLE OF DISPLAY SCREEM **

VDP LD B,29 ;SYNCRONIZE DISPLAY

CALL DLY1 ;

LD DE,(OFFST) ;FIND THE START OF THE MAIN DISPLAY

LD A,E ;IF OFFSET IS ZERO,

OR D ;

JR Z VDP1 ;THEN DISPLAY D-FILE

LD HL,(4010) ;ELSE FIND START OF VARIABLES

ADD HL,DE ;ADD THE OFFSET

LD DE,0005 ;ADD A$ HEADER LENGTH

JR VDP2 ;AND DISPLAY VFILE

VDP1 LD HL,(400C) ;LOAD DFILE START

ADD HL,DE ;DELAY

LD DE,0001 ;SKIP LEADING N/L

JR NZ VDP2 ;DELAY

VDP2 ADD HL,DE ;TRUE START OF DISPLAY

LD A,(LINES) ;CALCULATE NUMBER OF LINES

AND 1F ;

LD B,A ;SAVE IN B

RLCA ;CALCULATE BALNK LINES

RLCA ;

ADD A,3B ;

EX AF ;SAVE IN AF’ FOR NMI

LD C,08 ;

JR ZDP2 ;GO DISPLAY MAIN SCREEN ALREADY

;** BOTTOM OF DISPLAY SCREEN **

ZDP LD B,73 ;VSYNC DELAY

CALL DLY5 ;

LD A,(FLAG) ;DISPLAY ZX COMMAND LINE 24 (?)

BIT 2,A ;

JR NZ ZDP0 ;YES, GO DISPLAY LINE 24

EX AF ;NO LINE 24 = ADD EXTRA BLANK TIME

SUB 09 ;

EX AF ;SAVE FOR NMI

RET ;

ZDP0 XOR A ;START OF LINE 24 CAN BE FOUND BY

LD HL,(4010) ;START OF VAR AREA

LD DE,0021 ;32 CHARACTERS PLUS NEWLINE

SBC HL,DE ;SUBTRACT TO POINT TO START OF LINE 24

ZDP1 LD BC,0108 ;ONE ROW OR 8 HORIZONTAL LINES

ZDP2 SET 7,H ;ECHO DISPLAY ABOVE 32K

LD A,DD ;34 CHARACTERS PER LINE OR N/L (HALT WITH A6 INTERRUPT)

JP 0041 ;GO TO ZX INTERRUPT ROUTINE AND RETURN

;CONVERT LINE NUMBER IN (4007) TO DECIMAL

TRCE LD A,(FLAG) ;CHECK FLAG FOR BASIC LINE TRACE ON (?)

BIT 0,A ;

RET Z ;RETURN IF NO TRACE

LD DE,TRCL ;START OF TOP STATUS LINE

LD HL,(4007) ;BASIC LINE NUMBER

LD BC,FC18 ;DIVIDE BY 1000

CALL TR1 ;

LD BC,FF9C ;DIVIDE BY 100

CALL TR1 ;

LD BC,FFF6 ;DIVIDE BY 10

CALL TR1 ;

LD A,1C ;PLUS UNITS

ADD A,L ;

JR TR3 ;

TR1 LD A,1C ;DECIMAL CONVERSION HAS

TR2 ADD HL,BC ;VARIABLE EXECUTION TIME

INC A ;SO RUN WHILE NMI IS ON

JR C TR2 ;SUBTRACT BC UNTIL OVERFLOW

SBC HL,BC ;SUBTRACT ONE LOOP

DEC A ;ADJUST DIGIT

TR3 AND 3F ;LIMIT RESULT

LD (DE),A ;LOAD DIGIT IN TRACE FIELD

INC DE ;NEXT DIGIT

RET ;

;UPDATE 100 HR TIMER

FRM LD DE,TITL-1 ;POINTER TO TIMER FIELD LEAST SIGNIFICANT DIGIT

LD HL,FRM6+1 ;POINTER TO DIGIT LIMITS AND COLON FLAG

LD B,08 ;8 DIGITS

SCF ;FIXED EXECUTION TIME BETWEEN STATUS LINE AND VDP

FRM1 LD A,(DE) ;GET DIGIT

ADC A,00 ;CARRY FLAG ADDS ONE TO DIGIT

CP (HL) ;CHECK AGAINST LIMIT

CCF ;IF LIMIT THEN RESET DIGIT TO ZERO

JR C FRM2 ;AND CARRY INCREMENTS NEXT DIGIT

LD (DE),A ;ELSE NO CARRY AND LOAD TIMER FIELD WITH DIGIT+1

JR FRM3 ;

FRM2 LD A,1C ;SET DIGIT TO ZERO

LD (DE),A ;LOAD TIMER FIELD WITH ZERO DIGIT

FRM3 DEC DE ;

DEC HL ;

BIT 7,(HL) ;TEST FOR COLON FLAG

JR Z FRM5 ;JR IF NOT COLON

DEC DE ;POINT TO NEXT DIGIT

DEC HL ;POINT TO NEXT LIMIT

FRM5 DJNZ FRM1 ;DO EIGHT TIMES

RET ;

26 26 80 ; LIMIT 99: (HRS)

22 26 80 ; LIMIT 59: (MIN)

22 26 80 ; LIMIT 59: (SEC)

FRM6 22 26 ; LIMIT 59 (1/60SEC)

;INITIALIZE THE COMVAR STRING ARRAY

DIM1 XOR A ;USING DIM ?$ (USR 32671)

LD HL,(4016) ;USE CH-AD TO POINT TO NAME

LD DE,000F ;IN THE BASIC LINE

SBC HL,DE ;

LD A,(HL) ;GET NAME

LD HL,ATFR ;SAVE NAME IN ATFR

LD (HL),A ;

LD C,20 ;USR MUST RETURN 32

LD B,00 ;AS THE DIMENSION OF THE ARRAY

RET ;

ULD1 CALL ALD ;LOAD DATA FROM COMVAR TO STATUS LINE

JR DLD2 ;GO TRANSFER

DLD1 CALL ALD ;LOAD DATA FROM STATUS LINE TO COMVAR

EX DE,HL ;

DLD2 LDIR ;ACTUAL TRANSFER

JR ALD1 ;

ALD LD HL,(4016) ;SAVE THE CH-ADD POINTER

EX (SP),HL ;

PUSH HL ;

LD HL,ATFR ;POINT TO COMVAR NAME

CALL 004D ;SET CH-ADD TO COMVAR NAME

CALL 111C ;LOOK UP COMVAR ADDRESS

LD DE,0006 ;SKIP COMVAR HEADER

ADD HL,DE ;

LD DE,RTCL ;POINTER TO TIMER/CLOCK FIELD

LD BC,000B ;LENGTH OF TIMER/CLOCK FIELD

RET ;

ALD1 POP HL ;RESTORE CH-ADD

LD (4016),HL ;

RET ;

;CHECK IF OFFSET EXCEEDS VFIEL

SOFT1 LD HL,(4016) ;SAVE CH-ADD

PUSH HL ;

RST 20 ;FIRST CHECK IF A$ EXISTS

LD HL,(4010) ;EXIT TO DFILE IF A$ WAS ERASED

LD A,(HL) ;BECAUSE OF RUN, CLEAR, ETC.

CP C6 ;

JR NZ SOFT2 ;

CALL 1300 ;NOW FIND END OF ARRAY

LD HL,0340 ;ASSUME A FULL SCREEN IS REQUIRED

EX DE,HL ;CAUSE LINES CAN BE ADJUSTED

SBC HL,DE ;ENOUGH ROOM FOR A FULL SCREEN?

JR C SOFT2 ;

CALL 12DD ;EVALUATE THE USR EXPRESSION

JR NC SOFT3 ;AND SAVE IN BC

SOFT2 LD BC,0000 ;SET BC TO ZERO

SOFT3 LD (OFFST),BC ;SAVE IN OFFSET

JR ALD1 ;

JPTBL ;START OF PROGRAM VARIABLE AREA

SPARE "12345" ;SPARE BYTES

FLAG 04 ;FLAG CONTROLS FEATURES (eg COMMAND LINE on)

LINES 18 ;NUMBER OF DISPLAYED LINES

OFFST 00 ;OFFSET FROM START OF A$ VARIABLE

00 ;OR “0000” IS DFILE

USR RET ;3 BYTE USER CALL TO SHORT ROUTINE

00 ;MUST RETURN BEFORE TIME IS UP

00 ;

ATFR "Z$" ;NAME OF COMVAR (TRANSFERS DATA TO LINE 1)

RTCL "00:00:00:00" ;THESE THREE FIELDS MAKE UP STATUS LINE 1

TITL " NOVA 1000 V1.0";

TRCL " <" ;

;JUMP TABLE FOR NOVA ROUTINES

STRT JP STRT1 ;ACTIVATE NOVA VIDEO ROUTINE

DIM JP DIM1 ;INITIALIZE COMVAR NAME

ULD JP ULD1 ;MOVE COMVAR TO STATUS LINE

DLD JP DLD1 ;MOVE STATUS LINE TO COMVAR

SOFT JP SOFT1 ;TEST AND LOAD OFFSET

NOVA BASIC DEMO PROGRAM

Revision History

1986 – first release of NOVA – minimal documentation

18/04/2005 – NOVA resurrected – full annotated documentation and source program

19/04/2005 – EXIT methods added and FRAMES now updates - Thanks to Siegfried Engel