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The periodic routines & 
   The +D default variables and tables 

THE 'KEY-SCAN' ROUTINE
This is another entry point to the +D system; it is paged in when the Z80 reaches
address #028E, that is, the Spectrum 'KEY-SCAN' routine. So whenever the Spectrum tries
to scan the keyboard by calling this routine the +D is paged in first.

028E KEYSCAN    LD   L,#2F               Same instruction as in 'main' ROM.
0290            CALL #0296,KEYSC_1
0293            NOP                      Page-out into the 'main' ROM
0294            OUT  (231),A             'KEY_SCAN'.

0296 KEYSC_1    CALL #02A2,KEYSC_2
0299            LD   L,#2F               Restore registers for 'main' ROM
029B            LD   DE,#FFFF            'KEY-SCAN'
029E            LD   BC,#FEFE
02A1            RET

02A2 KEYSC_2    LD   A,(#3DE5)
02A5            CP   #47                 If a CALBAS was under execution return
02A7            RET  Z                   immediately to 'main' ROM 'KEY-SCAN'.
02A8            LD   A,(#3DE4)
02AB            CP   #58
02AD            JP   Z,#040C,TAKE_PRTR   Jump if this is a minimal system.
02B0            CP   #44                 Initialize the minimal system if there
02B2            JR   NZ,NOSYS            is no system file loaded.
02B4            CALL #2098,JKSCAN        Otherwise call the appropriate system
02B7            JP   #040C,TAKE_PRTR     routine and exit refreshing the 'P'
                                         channel.

02BA NOSYS      LD   HL,#2000
02BD            LD   BC,8192             Clear the 8K RAM.
02C0 CLR_RAM    LD   (HL),0
02C2            INC  HL
02C3            DEC  BC
02C4            LD   A,B
02C5            OR   C
02C6            JR   NZ,#02C0,CLR_RAM
02C8            OUT  (239),A             Clear control port.
02CA            OUT  (247),A             Clear printer port.
02CC            LD   HL,#02F2            Copy the default system variables and
02CF            LD   DE,#2000            settings to RAM.
02D2            LD   BC,104
02D5            LDIR
02D7            LD   A,#58               Signal 'minimal' system
02D9            LD   (#3DE4),A
02DC            LD   HL,275              Disk buffer offset.
02DF            LD   (#3AD2),HL
02E2            RET

02E3            PUSH BC                  Leftover routine?
02E4            PUSH DE
02E5            PUSH HL
02E6            CALL #02A2,KEYSC_2
02E9            POP  HL
02EA            INC  HL
02EB            LD   (23672),HL
02EE            POP  DE
02EF            POP  BC
02F0            LD   A,H
02F1            RET

THE 'SYSTEM VARIABLES'
These variables hold various default settings for drives etc. They can be accessed from
BASIC with POKE @p,n. Where p is 0 for RBCC at #2000. The default variables and system
tables, addresses #02F2-#0359, are copied to #2000 if there is no system file loaded. 

02F2 RBCC       DEFB #00       @0        No flashing borders.
02F3 TRAKS1     DEFB 80+128    @1        Drive 1 80 tracks double sided.
02F4 TRAKS2     DEFB 80+128    @2        Drive 2 80 tracks double sided.
02F5 STPRAT     DEFB 12        @3        "Steprate" 0 msec.
02F6 NSTAT      DEFB 1         @4        Network off.
02F7 WIDTH      DEFB 80        @5        Printer right margin.
02F8 PCODE      DEFB 0         @6        Expand tokens, etc. before printing.
02F9 LSPCE      DEFB 12        @7        Line spacing 12/72 inch.
02FA LFEED      DEFB 0         @8        Number of line feeds after CR 1.
02FB LMARG      DEFB 0         @9        Left margin at 0.
02FC GRAPH      DEFB 1         @10       Print graphic images of some chars.
02FD ZXPNT      DEFB 1         @11       +D printer port not used.
02FE RESERVED   DEFW #0000     @12
0301 ONERR      DEFW #0000     @14       Address of routine called after an
                                         error has occurred.
0303 EVERY_INT  DEFW #0000     @16       Address of routine called at every
                                         interrupt.

THE 'PRINTER CODES' TABLE
Here the default printer control codes are stored.
 
0304 INIT_PRT   DEFB 13,#80,#80,#80
0308            DEFB #80,#80,#80,#80
030C CHAR_PITCH DEFB 27,"M",#80,#80
0310            DEFB #80,#80,#80,#80
0314 N/72_LSPC  DEFB 27,"A",#80,#80
0318            DEFB #80,#80,#80,#80
031C GRAPH_DPI  DEFB 27,"*",5,#80
0320            DEFB #80,#80,#80,#80
0324 INIT_PRT2  DEFB #80,#80,#80,#80
0328            DEFB #80,#80,#80,#80

THE 'GRAPHIC REPRESENTATION' TABLE
This table consists of the graphic representations of the , # and (c) signs. The
'GRAPH' system variable (@10) determines whether the normal code or the graphic
representation is outputted to the printer. 

032C -SIGN     DEFB %00011000
032D            DEFB %00100000
032E            DEFB %00100000
032F            DEFB %01111000
0330            DEFB %00100000
0331            DEFB %00100000
0332            DEFB %01111100
0333            DEFB %00000000

0334 #-SIGN     DEFB %00000000
0335            DEFB %00100100
0336            DEFB %01111110
0337            DEFB %00100100
0338            DEFB %00100100
0339            DEFB %01111110
033A            DEFB %00100100
033B            DEFB %00000000

033C (c)-SIGN     DEFB %01111110
033D            DEFB %10000001
033E            DEFB %10111101
033F            DEFB %10100001
0340            DEFB %10100001
0341            DEFB %10111101
0342            DEFB %10000001
0343            DEFB %01111110

THE 'GREYSCALE' PRINTER CONTROL CODE
This code is outputted to the printer if a 'SCREEN$ 2' screendump is wanted.

0344            DEFB 27,"*",5,64
0348            DEFB 2,#80,#80,#80

THE 'GREYSCALE' TABLE
This table consists of three times three bytes of greyscale info. Each screen pixel is
converted into 3x3 printer dots during greyscale printing. The printer dots are ordered
as follows: The first dot row is produced from the first three bytes by taking the bit,
which number is corresponding with the colour number, from each of the three bytes. E.g.
for colour 2 (=red) bit 2 is used. In the same way the second and third dot rows are
produced from the second and third three bytes respectively.
The following eight 3x3 matrices are produced:

 White  Yellow  Cyan  Green Magenta  Red    Blue  Black
  000    000    100    000    100    010    110    111
  000    010    010    101    111    111    111    111
  000    000    001    000    001    010    011    111

Note that some greytones aren't right. E.g. cyan is darker than green, which isn't so on
the screen.
 
                       colour
                      76543210


034C            DEFB %00101011
034D            DEFB %00011111           first row
034E            DEFB %00000001

034F            DEFB %00000111
0350            DEFB %01101111           second row
0351            DEFB %00000111

0352            DEFB %00000001
0353            DEFB %00011111           third
0354            DEFB %00101011

0355 RAM2063    DEFW #1429,P_ALL         Address of 'output' routine for "P".
0357 RAM2065    DEFB #00                 The 'ENTER' flag.
0358 RAM2066    DEFW #0000               D_ERR_SP

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