Bob’s Notebook: TS2068 Interrupts

The Z80 has three Interrupt modes (IM) two of which are of special interest to the programmer. An “interrupt” is a signal sent to the microprocessог, which generally suspends the execution of the current program without the program being aware of it.

IM1 is the normal operating mode for the TS2068 and when an interrupt occurs, control is passed to address 56 (36h) in the ROM. This ROM routine updates the clock (FRAMES) and reads the keyboard. On exit from the routine, control is passed back to the exact place where the interrupt occurred. These interrupts take place at the rate of 60 per second.

IM2 allows us to divert interrupts to a user-generated routine and thus provides us with a powerful facility with almost limitless uses. This mode is called a vectored interrupt.

When the Z80 receives an IM2 interrupt, it expects the interrupting device (the TS2068 ULA) to place one byte of data on the data bus. This byte acts as the lou order of an address; the high order comes from the I register. These two bytes are concatenated to form an address and the Z80 looks at the contents of this address for a secопd address to which control is vectored.

The problem with the TS2068 is that the low order byte supplied to the Z80 varies from 0 to 255 and is not just 255 (FFh) at we аге told happens with the Spectrum. Опе technique to get around this is to create a block of addresses each filled with the same byte. The I register is changed to point to this block and IM2 is called. No matter what low order byte is supplied to the Z80, the block will produce the same address which will vector control to the start of our user-generated routine.

Meanwhile, the computer is able to carry out its main program as normal and the special routine is also acted upon so that in effect two programs аге operating in tandem. This allows for some interesting applications: digital clock; automatic line numbering; creation of function keys; constant read out of memory left; tracing of BASIC programs; disabling NEW; smooth sprite movement, to name a few.

Provided below is a program with one of these applications: automatic line numbering. Type in listing 2 and run it.

Now RANDOMIZE USR 65024 to create the special block of IM2 vector addresses. Use RANDOMIZE USR 65120 to start the autoline feature and USR 65124 to stop it. If you use NEW, RANDOMIZE USR 65024 and USR 65120 again before proceeding.

Clear 64255 before using the program.

The line number stepping is set to 10; you can change this with POKE 65192, s, where s is the step value.

If the line number exceeds 9999, a colon will be printed in the first position to warn that the line cannot be entered.

Be sure to turn the program off before overwriting any of the code used by the program.

Listing 1 generates the block of bytes required for the IM2 vectoring.

Listing 2 is the assembly language version of the AUTOLINE code.

Listing 1

FE00 C5               PUSH BC
FE01 D5               PUSH DE
FE02 E5               PUSH HL
FE03 F5               PUSH AF
FE04 2100FB           LD HL,FB00
FE07 0600             LD B,00
FE09 36FC             LD (HL),FC
FE0B 23               INC HL
FE0C 10FB             DJNZ FE09
FE0E 36FC             LD (HL),FC
FE10 3EC3             LD A,C3
FE12 32FCFC           LD (FCFC),A
FE15 2169FE           LD HL,FE69
FE18 22FDFC           LD (FCFD),HL
FE1B 3EFB             LD A,FB
FE1D ED47             LD I,A
FE1F F1               POP AF
FE20 E1               POP HL
FE21 D1               POP DE
FE22 C1               POP BC
FE29 C9               RET

Listing 2

FE60 ED5E             IM 2
FE62 C9               RET
FE63 00               NOP
FE64 ED56             IM 1
FE66 C9               RET
FE67 00               NOP
FE68 00               NOP
FE69 FF               RST 38
FE6A F3               DI
FE6B F5               PUSH AF
FE6C E5               PUSH HL
FE6D D5               PUSH DE
FE6E C5               PUSH BC
FE6F 3A68FE           LD A,(FE68)
FE72 FE00             CP 00
FE74 2027             JR NZ,FE9D
FE76 3A825C           LD A,(5C82)
FE79 FE20             CP 20
FE7B 2071             JR NZ,FEEE
FE7D 3A835C           LD A,(5C83)
FE80 FE17             CP 17
FE82 206A             JR NZ,FEEE
FE84 21085C           LD HL,5C08
FE87 7E               LD A,(HL)
FE88 FE0C             CP 0C
FE8A 2862             JR Z,FEEE
FE8C 21045C           LD HL,5C04
FE8F 7E               LD A,(HL)
FE90 FE0D             CP 0D
FE92 2804             JR Z,FE98
FE94 FEFF             CP FF
FE96 2056             JR NZ,FEEE
FE98 3E04             LD A,04
FE9A 3268FE           LD (FE68),A
FE9D 3A68FE           LD A,(FE68)
FEA0 3D               DEC A
FEA1 3268FE           LD (FE68),A
FEA4 2A495C           LD HL,(5C49)
FEA7 110A00           LD HE,000A
FEAA 19               ADD HL,DE
FEAB 0118FC           LD BC,FC18
FEAE CDD1FE           CALL FED1
FEB1 FE03             CP 03
FEB3 2839             JR Z,FEEE
FEB5 019CFF           LD BC,FF9C
FEB8 CDD1FE           CALL FED1
FEBB FE02             CP 02
FEBD 282F             JR Z,FEEE
FEBF 01F6FF           LD BC,FFF6
FEC2 CDD1FE           CALL FED1
FEC5 FE01             CP 01
FEC7 2825             JR Z,FEEE
FEC9 01FFFF           LD BC,FFFF
FECC CDD1FE           CALL FED1
FECF 181D             JR FEEE
FED1 AF               XOR A
FED2 09               ADD HL,BC
FED3 3C               INC A
FED4 38FC             JR C,FED2
FED6 ED42             SBC HL,BC
FED8 3D               DEC A
FED9 C630             ADD A,38
FEDB E5               PUSH HL
FEDC 21085C           LD HL,5C08
FEDF 77               LD (HL),A
FEE0 3A3B5C           LD A,(5C3B)
FEE3 CBEF             SET 5,A
FEE5 213B5C           LD HL,5C3B
FEE8 77               LD (HL),A
FEE9 E1               POP HL
FEEA 3A68FE           LD A,(FE68)
FEED C9               RET
FEEE C1               POP BC
FEEF D1               POP DE
FEEF E1               POP HL
FEF0 F1               POP AF
FEF2 FB               EI
FEF3 C9               RET

Listing 3

    1 REM AUTOLINE
  120 LET a=0: LET b=11: LET C=12: LET d=13: LET e=14: LET f=15
  130 LET line=1000
  140 LET address=65024+(line-1000)*6.4
  150 RESTORE line
  160 READ s$,sum
  170 LET tot=0
  180 LET byte=16*val s$(1)+val s$(2)
  190 LET tot=tot+byte
  200 POKE address,byte
  210 Let s$=s$(3 TO )
  220 LET address=address+1
  230 IF s$<>"" THEN GO TO 180
  240 POKE 23692,255
  250 IF sum=tot THEN PRINT "Line ";line;" OK.":LET line=line+1:GO TO 150
  260 PRINT "Error in line ";line
  270 BEEP .4,-10: Beep .4,-16
  280 STOP
  300 SAVE "AUTOLINE"
  310 STOP
 1000 DATA "C5D5E5F52100FB060036FC2310FB36FC3EC332FCFC2169FE22FDFC3EFBED47F1E1D1C1C900000000000000000000000000000000000000000000000000000000",5520
 1001 DATA "0000000000000000000000000000000000000000000000000000000000000000ED5EC900ED56C90000FFF3F5E5D5C53A68FEFE0020273A825CFE2020713A835C",4171
 1002 DATA "FE17206A21085C7EFE0C286221045C7EFE0D2804FEFF20563E043268FE3A68FE3D3268FE2A495C110A00190118FCCDD1FEFE032839019CFFCDD1FEFE02282F01",6612
 1003 DATA "F6FFCDD1FEFE01282501FFFFCDD1FE181DAF093C38FCED423DC630E521085C773A4B5CCBEF213B5C77E13A68FEC9C1D1E1F1FBC9000000000000000000000000",7492