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The Snapshot routines 

This piece of code determines whether the NMI button was pressed or a return from a
CALBAS was made.

0053 NMI_1      LD   A,(#3DE5)
0056            CP   #47                 Check if returning from a CALBAS.
0058            JR   NZ,#0060,NMI_2      Jump if not returning from a CALBAS.
005A            XOR  A
005B            LD   (#3DE5),A           Clear CALBAS executing.
005E            POP  AF                  Restore A register and return to the
005F            RET                      calling routine.
0060 NMI_2      LD   A,R
0062            PUSH AF                  Save R register and interrupt status.
0063            JR   #0069,SNAPSHOT
0065            DEFB #00

This is the third point, from address #0000, at which the +D system is paged in. It is
reached  either by pressing the 'snapshot' button or when returning from a CALBAS.

0066 NMI        PUSH AF                  Save A register.
0067            JR   #0053,NMI_1

When the 'snapshot' button is pressed this routine is executed. By pressing the keys 1
or 2 a choice can be made to dump the screen to the printer in small or grey-scale
format. As soon as the system file is loaded it is possible to make snapshots to disk
(See #20A1 and further).
0069 SNAPSHOT   LD   (#3FFE),SP          Use 'internal' stack.
006D            LD   SP,#3FFE
0070            LD   A,I
0072            PUSH AF                  Save all registers, starting with I.
0073            PUSH HL
0074            PUSH BC
0075            PUSH DE
0076            EX   AF,AF'              Save alternative registerset.
0077            EXX
0078            PUSH AF
0079            PUSH HL
007A            PUSH BC
007B            PUSH DE
007C            PUSH IX                  Save the index registers.
007E            PUSH IY
0080            DI                       Disable interrupts in case #0066 was
                                         called normally. When a NMI occurs the
                                         Z80 disables interrupts. 
0081            LD   HL,#00B1,SNAP_EXIT
0084            PUSH HL                  RETurn address from snapshot.
0085            LD   (#2066),SP          Store current stackpointer at D_ERR_SP
                                         so an error will activate SNAP_EXIT.
0089 SNAP_KEYS  LD   BC,#F7FE            Keyboard port and key 1-5 I/O address.
008C            IN   E,(C)
008E            BIT  0,E                 CASE key OF
0090            JP   Z,#12D2,COPS        1: jump to the smallcopy routine.
0093            BIT  1,E
0095            JP   Z,#133E,COPS2       2: jump to the greyscale routine.
0098            CALL #0527,SYSTEM_Z      System loaded?
009B            CALL Z,#20A1,JSNAP       Yes, then call system snap routine.
009E            INC  A                   END CASE.
009F            AND  #07
00A1            OUT  (C),A               Nice stripes in border.
00A3            LD   B,#FE               CAPS-V I/O address.
00A5            IN   E,(C)
00A7            BIT  2,E
00A9            JR   NZ,#0089,SNAP_KEYS  Test for X, loop if not pressed.
00AB SNAP_KEY1  IN   E,(C)
00AD            BIT  2,E
00AF            JR   Z,#00AB,SNAP_KEY1   Wait until X is released again.

This routine is used to return to the snapshotted program.

00B2            LD   HL,#0000            Clear D_ERR_SP.
00B5            LD   (#2066),HL
00B8            CALL #168E,BORD_REST
00BB            LD   SP,#3FEA
00BE            POP  IY                  Restore the index registers.
00C0            POP  IX
00C2            POP  DE                  Restore the alternate registerset.
00C3            POP  BC
00C4            POP  HL
00C5            POP  AF
00C6            EX   AF,AF'
00C7            EXX
00C8            CALL #011D,?_ROMBANK     Determine current 'main' ROM bank.
00CB            JR   NZ,#00F5,SNAP_EX3   Jump if 128K ROM bank.
00CD            CALL #0511,REST_PBUF
00D0            POP  DE                  Restore the registerset.
00D1            POP  BC
00D2            POP  HL
00D3            POP  AF
00D4            LD   I,A
00D6            CP   #00
00D8            JR   Z,#00E0,SNAP_EX1
00DA            CP   #3F                 When the I register doesn't contain #00
00DC            JR   Z,#00E0,SNAP_EX1    or #3F, it is most likely that the
00DE            IM   2                   interrupt mode is 2.
00E0 SNAP_EX1   LD   SP,(#3FFE)          Restore stack pointer.
00E4            POP  AF
00E5            LD   R,A                 Jump if the interupts were disabled
00E7            JP   PO,#00F1,SNAP_EX2   when 'SNAPSHOT' was entered.
00EA            PUSH HL
00EB            LD   HL,#004F            Otherwise return to snapshotted program
00EE            JP   #004F,UNPAGE_HL     via 'main' ROM "POP  HL", "POP  AF" and
00F1 SNAP_EX2   POP  AF                  Return to the instruction before which
00F2            JP   #0050,UNPAGE_1      the snapshot occurred.

This piece of code does almost the same as the above. The only difference is that it
returns to the 128 'main' ROM at a somewhat different address.

00F8            POP  DE
00F9            POP  BC
00FA            POP  HL
00FB            POP  AF
00FC            LD   I,A
00FE            CP   #00
0100            JR   Z,#0108,SNAP_EX4
0102            CP   #3F
0104            JR   Z,#0108,SNAP_EX4
0106            IM   2
0108 SNAP_EX4   LD   SP,(#3FFE)
010C            POP  AF
010D            LD   R,A
010F            JP   PO,#0119,SNAP_EX5
0112            PUSH BC
0113            LD   BC,#007B
0116            JP   #0046,UNPAGE_BC

0119 SNAP_EX5   POP  AF
011A            JP   #0047,UNPAGE_0

This routine determines, by examining the byte at address #0001, which bank of the 128K
ROM is selected. Because the +D system is currently paged in the 'main' ROM can't be
accessed directly. So a routine, which pages out the +D, is copied to the printer buffer.
But first the first 10 bytes of the printer buffer are saved.

011D ?_ROMBANK  LD   HL,23296            Save the 10 bytes needed by the
0120            LD   DE,#3BE6            subroutine in +D RAM.
0123            LD   BC,10
0126            LDIR
0128            LD   HL,#051D,DET_ROM    Copy the subroutine to the freed bytes.
012B            LD   DE,23296,DET_ROM'
012E            LD   BC,10
0131            LDIR
0133            LD   A,#47               Signal 'CALBAS executing' (the NMI
0135            LD   (#3DE5),A           routine returns immediately).
0138            JP   23296,DET_ROM'      Exit via DET_ROM.

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