Mastering Sideways ROM & RAM - Module 14 - Unrecognised Interupts
-----------------------------------------------------------------

  An interupt is a signal to the 6502 processor informing it that a
hardware device somewhere in the system requires immediate attention.
Interupts not recognised by the operating system are first offered to
the paged ROMs and then to the user via the IRQ2 vector.

  Service call 5 is used to offer unrecognised interupts to the paged
ROMs but, because an interupt could be generated by almost any device,
it is not possible to identify the source of an interupt without
directly interrogating the hardware that might be responsible for
generating it.

  If you want to design a SWR program to process interupts it must
intercept service call 5 and poll the hardware that it expects to
generate an interupt to see if that device was responsible for the
unrecognised interupt, and if so process it. If you do not want the
interupt to be offered to the other ROMs the accumulator should be
reset to zero before returning. Always return from an unrecognised
interupt with RTS and not RTI.

  One way of connecting hardware to the BBC Micro is to use the 6522
versatile interface adaptor (VIA). There are two 6522 VIAs in the BBC
B. VIA-A is used for internal functions and the joystick fire buttons.
VIA-B provides the parallel printer interface and the user port.
Printers are the most commonly used device connected to VIA-B, and a
printer buffer will be used to demonstrate how interupts generated by
external hardware can be intercepted and processed in sideways RAM.

  The program BUFFER is used to generate an object code file just
under &100 bytes long. When the object code is loaded into SWR it uses
the remaining 15.75K in the RAM bank as a parallel printer buffer.
When sideways RAM is used in this way the size of the object code
needs to be as small as possible. For this reason a number of
techniques have been used in the source code to make the object code
fit into just one page of memory.

  The SWR header used by the buffer is as small as it can be. All the
optional coding has been left out. There is no binary version number,
title string, version string, copyright message or tube relocation
address. Just the essential jump to the service entry point, ROM type
byte, copyright offset pointer and minimum copyright string (lines
250-340). Resetting the user print vector is done with the minimum of
coding by taking the Osbyte &A8 call out of the object code (lines
180-210). The service call interpreter does not implement * commands
to enable the buffer but instead intercepts service call &FE (tube
system post initialisation) to set up the user print extended vector
to point to the printer buffer code (lines 470-710).

  To use the buffer load the object code into SWR and press the Break
key. Service call &FE is always issued on reset and so the buffer will
be initialised every time you press Break.

  After pressing the Break key type *FX5,3 to enable the buffer. The
printer can then be used as normal but with a 15.75K buffer. Type
*FX5,1 or press Ctrl+Break to disable the buffer after printing has
been completed.

  The buffer was designed to be used with a parallel printer using
the Centronics.  It is unsuitable for serial or other printers and
I suspect that the early versions of the Solidisk sideways RAM boards
may not allow this program to work properly, although I have not been
able to test the program with one to confirm this suspicion.

  The user print vector is normally used to provide a printer driver
for printers which do not have either a Centronics parallel or a
standard serial interface. With this printer buffer program the user
print vector interfaces the printer buffer with the SWR buffer and the
printer hardware.

  The buffer program reads data from the printer buffer using Osbyte
&91 (lines 1020-1040) and writes it into the SWR buffer using a
pointer (line 1080) to indicate the position in SWR to store each
individual byte. The data is read from the SWR buffer using another
pointer (line 1540) to indicate the byte to be written into the VIA-B
printer output register. When a byte is recieved by the printer the
acknowledge line goes low and generates an IRQ interupt. This is the
source of the interupt processed by the program.

  The pointers are updated with every read and write of the SWR buffer
and when a pointer reaches &C000 it is reset to &8100 (lines 900-920
and lines 1620-1640). When the last character in the buffer has been
printed the printer driver declares itself dormant and the MOS is
informed using Osbyte &7B (lines 1350-1470). When all the data is
either printed or in the SWR buffer the computer is released to
continue with other tasks but you must not press the Break key or use
*FX5 until all printing has been completed. You can continue with any
task such as word processing or programming while the printer is using
the data in SWR printer buffer.

  The 6522 VIA used to control the parallel printer interface is quite
a complex device controled by 16 registers. The VIA-B registers are
mapped onto Sheila addresses &60 to &6F as shown in figure 14.1




      +-------------------------------------+
&FE6F ! A Input/Output (no handshake)       !
      +-------------------------------------+
&FE6E ! Interupt control                    !
      +-------------------------------------+
&FE6D ! Interupt status                     !
      +-------------------------------------+
&FE6C ! Peripheral control (handshake)      !
      +-------------------------------------+
&FE6B ! Auxiliary (timer/shift reg) control !
      +-------------------------------------+
&FE6A ! Shift register                      !
      +-------------------------------------+
&FE69 ! Timer 2 MSB counter                 !
      +-------------------------------------+
&FE68 ! Timer 2 LSB latch/counter           !
      +-------------------------------------+
&FE67 ! Timer 1 MSB latch                   !
      +-------------------------------------+
&FE66 ! Timer 1 LSB latch                   !
      +-------------------------------------+
&FE65 ! Timer 1 MSB latch/counter           !
      +-------------------------------------+
&FE64 ! Timer 1 LSB latch/counter           !
      +-------------------------------------+
&FE63 ! Direction register A                !
      +-------------------------------------+
&FE62 ! Direction register B                !
      +-------------------------------------+
&FE61 ! A Input/Output                      !
      +-------------------------------------+
&FE60 ! B Input/Output                      !
      +-------------------------------------+

Figure 14.1  The VIA-B registers
-----------  -------------------



  Both VIAs have 20 I/O lines grouped into an A side and a B side of
10 lines each. These 10 lines are divided into 8 data lines and 2
handshake lines. The data lines are called PA0 to PA7 on the A side
and PB0 to PB7 on the B side. The handshake lines are called CA1 and
CA2 on the A side and CB1 and CB2 on the B side. The BBC Micro uses
the A side of VIA-B for the printer interface and the B side of VIA-B
for the user port. PA0 to PA7 are used as the output data lines for
the parallel printer, CA1 is used for the printer acknowledge and CA2
for the printer strobe.

  The printer buffer program uses the output register at &FE61, the
peripheral control register at &FE6C, the interupt status register at
&FE6D and the interupt control register at &FE6E.

  The peripheral control register is used to set up the handshaking
with the printer (lines 1490-1510 and line 1550). The number &0A is
stored in the least significant nybble of this register and this has
the effect of sending a strobe pulse (CA2 low) to the printer for 1
cycle of the 1 MHz clock following a write to the output register at
&FE61. The Centronics interface requires a strobe pulse width of at
least 0.5 micro seconds at the recieving terminal. The peripheral
control register is shown in figure 14.2 with bits 1 and 3 set (see
also program lines 1490-1510 and line 1550).






    7       6       5       4       3       2       1       0
+-------+-------+-------+-------+-------+-------+-------+-------+
!          CB2          !  CB1  !          CA2          !  CA1  !
!        control        !control!        control        !control!
+-------+-------+-------+-------+-------+-------+-------+-------+
                                    1       0       1       0     &0A

Figure 14.2  The peripheral control (handshake) register.
-----------  --------------------------------------------



  The acknowledge line is connected to CA1 and is used to generate an
interupt when the printer has recieved data and is ready to accept
more data from the printer buffer. The Centronics protocol takes the
acknowledge line low for about 12 micro seconds. CA1 is reset to zero
in the peripheral control register (lines 1490-1510 and line 1550) and
its corresponding bit in the interupt status register will be set to 1
when the voltage on the CA1 input line goes from high to low. CA1 will
set a bit in the interupt status register and generate an IRQ interupt
if the CA1 bit in the status register has been set. Bit 2 (CA1) is set
by setting bits 7 and 2, ie. by storing &82 in the control register as
shown in figure 14.3 (line 1520 and line 1560). Bit 2 of the control
register can be cleared by clearing bit 7 and setting bit 2, ie. by
storing 2 in the control register (lines 1420-1430).





     7       6     5     4     3     2     1     0
+---------+-----+-----+-----+-----+-----+-----+-----+
!   IRQ   ! T1  ! T2  ! CB1 ! CB2 ! SR  ! CA1 ! CA2 ! Interupt status
+---------+-----+-----+-----+-----+-----+-----+-----+
!Set/Clear! T1  ! T2  ! CB1 ! CB2 ! SR  ! CA1 ! CA2 ! Interupt control
+---------+-----+-----+-----+-----+-----+-----+-----+
     1       0     0     0     0     0     1     0    &82

Figure 14.3  Interupt status and control registers.
-----------  --------------------------------------




 The control and status registers work as a pair and the status
register is interrogated by the buffer program to see if the printer
is the source of an interupt (lines 1140-1160). If bits 1 and 7 of the
status register are not set then the printer was not the source of the
interupt and control is returned to the MOS with all the register
preserved (line 1160, lines 1280-1300 and lines 640-710). If the
buffer has not been enabled with *FX5,3 control is also returned to
the MOS (lines 1170-1190, lines 1280-1300 and lines 640-710).




   10 REM: BUFFER
   20 REM: (C) Gordon Horsington 1987
   30 MODE7
   40 HIMEM=&3C00
   50 DIM save 50
   60 diff=&8000-HIMEM
   70 ROMnumber=&F4
   80 stack=&103
   90 uptv=&222
  100 starfx5=&285
  110 pointer=&8100
  120 printout=&FE61
  130 handshake=&FE6C
  140 status=&FE6D
  150 control=&FE6E
  160 osbyte=&FFF4
  170 oscli=&FFF7
  180 A%=&A8
  190 X%=0
  200 Y%=&FF
  210 extvec=(USR(osbyte)AND&FFFF00)DIV256
  220 FOR pass = 0 TO 2 STEP 2
  230 P%=HIMEM
  240 [       OPT pass
  250         BRK
  260         BRK
  270         BRK
  280         JMP service+diff
  290         OPT FNequb(&82)
  300         OPT FNequb((copyright+diff) MOD 256)
  310 .copyright
  320         BRK
  330         OPT FNequs("(C)")
  340         BRK
  350 .flag
  360         BRK
  370 .service
  380         PHA
  390         TXA
  400         PHA
  410         TYA
  420         PHA
  430         TSX
  440         LDA stack,X
  450         CMP #5
  460         BEQ interupt
  470         CMP #&FE
  480         BNE exit
  490         LDA #(buffer+diff) MOD 256 
  500         STA extvec+&33
  510         LDA #(buffer+diff) DIV 256 
  520         STA extvec+&34
  530         LDA ROMnumber
  540         STA extvec+&35
  550         LDA #&81
  560         STA datin+diff+1
  570         STA datin+diff+2
  580         STA datout+diff+1
  590         STA datout+diff+2
  600         LDX #&33
  610         LDY #&FF
  620         STX uptv
  630         STY uptv+1
  640 .exit
  650         PLA
  660         TAY
  670         PLA
  680         TAX
  690         PLA
  700 .return
  710         RTS
  720 .buffer
  730         CPY #3
  740         BNE return
  750         CMP #1
  760         BNE active
  770         STA flag+diff
  780 .active
  790         PHA
  800         TXA 
  810         PHA 
  820         TYA
  830         PHA
  840 .outerloop
  850         LDY datin+diff+2
  860         LDX datin+diff+1
  870         INX 
  880         BNE nottop
  890         INY 
  900         CPY #&C0
  910         BNE nottop
  920         LDY #&81
  930 .nottop
  940         CPX datout+diff+1
  950         BNE morespace
  960         CPY datout+diff+2
  970         BEQ innerloop
  980         SEC
  990 .morespace
 1000         TYA 
 1010         PHA 
 1020         LDA #&91
 1030         LDX #3
 1040         JSR osbyte
 1050         PLA 
 1060         BCS innerloop
 1070 .datin
 1080         STY pointer
 1090         INC datin+diff+1
 1100         STA datin+diff+2
 1110         CLC 
 1120         BCC innerloop
 1130 .interupt
 1140         LDA #&82
 1150         BIT status
 1160         BEQ restore
 1170         LDA starfx5
 1180         CMP #3
 1190         BNE restore
 1200 .innerloop
 1210         LDA flag+diff
 1220         BNE notpoll
 1230         LDA #&82
 1240         BIT status
 1250         BNE notpoll
 1260 .step
 1270         BCC outerloop
 1280 .restore
 1290         CLC
 1300         BCC exit
 1310 .notpoll
 1320         PHP 
 1330         LDA #0
 1340         STA flag+diff
 1350         LDA datin+diff+1
 1360         CMP datout+diff+1
 1370         BNE output
 1380         LDA datin+diff+2
 1390         CMP datout+diff+2
 1400         BNE output
 1410         PLP
 1420         LDX #2
 1430         STX control
 1440         LDA #&7B
 1450         INX
 1460         JSR osbyte
 1470         JMP restore+diff
 1480 .output
 1490         LDA handshake
 1500         AND #&F0
 1510         ORA #&0A
 1520         LDX #&82
 1530 .datout
 1540         LDY pointer
 1550         STA handshake
 1560         STX control
 1570         STY printout
 1580         INC datout+diff+1
 1590         BNE notpage
 1600         LDY datout+diff+2
 1610         INY 
 1620         CPY #&C0
 1630         BNE moreroom
 1640         LDY #&81
 1650 .moreroom
 1660         STY datout+diff+2
 1670 .notpage
 1680         PLP 
 1690         BCS innerloop
 1700         BCC step
 1710 .lastbyte
 1720 ]
 1730 NEXT
 1740 INPUT'"Save filename = "filename$
 1750 IF filename$="" END
 1760 $save="SAVE "+filename$+" "+STR$~(HIMEM)+" "+STR$~(las
      tbyte)+" FFFF8000 FFFF8000"
 1770 X%=save
 1780 Y%=X% DIV 256
 1790 *OPT1,2
 1800 CALL oscli
 1810 *OPT1,0
 1820 END
 1830 DEFFNequb(byte)
 1840 ?P%=byte
 1850 P%=P%+1
 1860 =pass
 1870 DEFFNequw(word)
 1880 ?P%=word
 1890 P%?1=word DIV 256
 1900 P%=P%+2
 1910 =pass
 1920 DEFFNequd(double)
 1930 !P%=double
 1940 P%=P%+4
 1950 =pass
 1960 DEFFNequs(string$)
 1970 $P%=string$
 1980 P%=P%+LEN(string$)
 1990 =pass