No-Slot-Clock (Dallas SmartWatch DS1216) GEOS driver "NoSlot Clock".
Partial disassembly and comments by Nick Westgate (and CiderPress).

Note: This code requires the clock to be in the internal C8 ROM space. (CD or CF ROM sockets.)

0800-   4C 15 08    JMP   $0815    ;read time
0803-   4C B8 08    JMP   $08B8
0806-   4C BC 08    JMP   $08BC    ;write time
0809-   4C E4 08    JMP   $08E4
080C-   4C F1 08    JMP   $08F1
080F-   4C 12 08    JMP   $0812
0812-   A2 30       LDX   #$30
0814-   60          RTS
0815-   A9 00       LDA   #$00
0817-   10 37       BPL   $0850
0819-   A9 3B       LDA   #$3B
081B-   8D 16 08    STA   $0816
081E-   20 6E 08    JSR   $086E    ;init clock
0821-   20 93 08    JSR   $0893    ;get clock data byte (0, discard centiseconds)
0824-   A2 03       LDX   #$03     ;(4 bytes)
0826-   20 93 08    JSR   $0893    ;get clock data byte (1-4 -> F202-F205: day h m s)
0829-   9D 02 F2    STA   $F202,X
082C-   CA          DEX
082D-   10 F7       BPL   $0826
082F-   A2 02       LDX   #$02     ;(3 bytes, this overwrites day at F202)
0831-   20 93 08    JSR   $0893    ;get clock data byte (5-7 -> F200-F202: Y M D)
0834-   9D 00 F2    STA   $F200,X
0837-   CA          DEX
0838-   10 F7       BPL   $0831
083A-   2C 06 F2    BIT   $F206
083D-   10 11       BPL   $0850
083F-   AD 03 F2    LDA   $F203
0842-   C9 FF       CMP   #$FF
0844-   D0 0A       BNE   $0850
0846-   AD 04 F2    LDA   $F204
0849-   C9 FF       CMP   #$FF
084B-   D0 03       BNE   $0850
084D-   4E 06 F2    LSR   $F206
0850-   4C DB 08    JMP   $08DB    ;exit safely

0853-   A9 00       LDA   #$00     ;*** write 0 byte to clock
0855-   84 09       STY   $09      ;*** write byte to clock
0857-   38          SEC
0858-   A0 FF       LDY   #$FF
085A-   C8          INY
085B-   E9 0A       SBC   #$0A
085D-   10 FB       BPL   $085A
085F-   69 0A       ADC   #$0A
0861-   85 07       STA   $07
0863-   98          TYA
0864-   0A          ASL
0865-   0A          ASL
0866-   0A          ASL
0867-   0A          ASL
0868-   05 07       ORA   $07
086A-   A0 00       LDY   #$00
086C-   F0 11       BEQ   $087F

086E-   78          SEI            ;*** init clock
086F-   AD 15 C0    LDA   $C015    r:RDCXROM
0872-   85 0F       STA   $0F
0874-   8D 07 C0    STA   $C007    w:SETINTCXROM
0877-   AD 04 C8    LDA   $C804
087A-   A0 07       LDY   #$07
087C-   B9 F7 08    LDA   $08F7,Y
087F-   38          SEC
0880-   6A          ROR
0881-   A2 00       LDX   #$00
0883-   90 02       BCC   $0887
0885-   A2 01       LDX   #$01
0887-   DD 00 C8    CMP   $C800,X
088A-   4A          LSR
088B-   D0 F4       BNE   $0881
088D-   88          DEY
088E-   10 EC       BPL   $087C
0890-   A4 09       LDY   $09
0892-   60          RTS

0893-   A0 07       LDY   #$07     ;*** get clock data byte (convert BCD to hex)
0895-   AD 04 C8    LDA   $C804
0898-   6A          ROR
0899-   66 07       ROR   $07
089B-   88          DEY
089C-   10 F7       BPL   $0895
089E-   A5 07       LDA   $07
08A0-   A8          TAY
08A1-   29 0F       AND   #$0F
08A3-   85 07       STA   $07
08A5-   98          TYA
08A6-   4A          LSR
08A7-   4A          LSR
08A8-   4A          LSR
08A9-   4A          LSR
08AA-   18          CLC
08AB-   A8          TAY
08AC-   F0 07       BEQ   $08B5
08AE-   A9 00       LDA   #$00
08B0-   69 0A       ADC   #$0A
08B2-   88          DEY
08B3-   D0 FB       BNE   $08B0
08B5-   65 07       ADC   $07
08B7-   60          RTS
08B8-   CE 16 08    DEC   $0816
08BB-   60          RTS

08BC-   20 6E 08    JSR   $086E    ;init clock
08BF-   20 53 08    JSR   $0853    ;write 0 byte to clock
08C2-   A0 02       LDY   #$02
08C4-   B9 03 00    LDA   $0003,Y
08C7-   20 55 08    JSR   $0855    ;write byte to clock
08CA-   88          DEY
08CB-   10 F7       BPL   $08C4
08CD-   20 53 08    JSR   $0853    ;write 0 byte to clock
08D0-   A0 02       LDY   #$02
08D2-   B9 00 00    LDA   $0000,Y
08D5-   20 55 08    JSR   $0855    ;write byte to clock
08D8-   88          DEY
08D9-   10 F7       BPL   $08D2

08DB-   26 0F       ROL   $0F      ;prepare for exit
08DD-   B0 03       BCS   $08E2
08DF-   8D 06 C0    STA   $C006    w:SETSLOTCXROM
08E2-   58          CLI
08E3-   60          RTS

08E4-   A5 00       LDA   $00
08E6-   8D 43 08    STA   $0843
08E9-   A5 01       LDA   $01
08EB-   8D 4A 08    STA   $084A
08EE-   A2 80       LDX   #$80
08F0-   2C A2 00    BIT   $00A2
08F3-   8E 06 F2    STX   $F206
08F6-   60          RTS

08F7-   5C A3 3A C5 5C A3 3A C5    ;*** clock pattern

08FF-   59
0900-   FA          PLX
0901-   88          DEY
0902-   10 F8       BPL   $08FC
0904-   20 B7 5C    JSR   $5CB7
0907-   68          PLA
0908-   C9 0F       CMP   #$0F
090A-   D0 03       BNE   $090F
090C-   4C 4C 71    JMP   $714C
090F-   C9 80       CMP   #$80
0911-   90 20       BCC   $0933
0913-   C9 8E       CMP   #$8E
0915-   B0 1C       BCS   $0933
0917-   20 D7 65    JSR   $65D7
091A-   C9 81       CMP   #$81
091C-   D0 03       BNE   $0921
091E-   4C D9 93    JMP   $93D9
0921-   C9 82       CMP   #$82
0923-   F0 04       BEQ   $0929
0925-   C9 8A       CMP   #$8A
0927-   D0 03       BNE   $092C
0929-   4C 6E 71    JMP   $716E
092C-   C9 83       CMP   #$83
092E-   D0 0A       BNE   $093A
0930-   4C 7B 71    JMP   $717B
0933-   C9 FF       CMP   #$FF
0935-   D0 03       BNE   $093A
0937-   4C EA AB    JMP   $ABEA
093A-   20 40 71    JSR   $7140
093D-   A2 00       LDX   #$00
093F-   60          RTS
0940-   A9 00       LDA   #$00
0942-   8D F8 70    STA   $70F8
0945-   A2 72       LDX   #$72
0947-   A9 DB       LDA   #$DB
0949-   4C 08 4D    JMP   $4D08
094C-   20 13 74    JSR   $7413
094F-   A9 00       LDA   #$00
0951-   85 AA       STA   $AA
0953-   20 91 5D    JSR   $5D91
0956-   20 81 03    JSR   $0381
0959-   8A          TXA
095A-   F0 03       BEQ   $095F
095C-   4C 64 5E    JMP   $5E64
095F-   A9 00       LDA   #$00
0961-   85 70       STA   $70
0963-   68          PLA
0964-   68          PLA
0965-   20 32 5F    JSR   $5F32
0968-   8A          TXA
0969-   D0 F1       BNE   $095C
096B-   4C 45 5F    JMP   $5F45
096E-   A9 FF       LDA   #$FF
0970-   48          PHA
0971-   A9 58       LDA   #$58
0973-   48          PHA
0974-   A9 00       LDA   #$00
0976-   85 00       STA   $00
0978-   4C 8D 03    JMP   $038D
097B-   A9 80       LDA   #$80
097D-   8D B1 61    STA   $61B1
0980-   20 D7 4D    JSR   $4DD7
0983-   A5 12       LDA   $12
0985-   18          CLC
0986-   69 01       ADC   #$01
0988-   85 08       STA   $08
098A-   A5 13       LDA   $13
098C-   69 00       ADC   #$00
098E-   85 09       STA   $09
0990-   A9 12       LDA   #$12
0992-   85 07       STA   $07
0994-   A9 85       LDA   #$85
0996-   85 06       STA   $06
0998-   A2 08       LDX   #$08
099A-   A0 06       LDY   #$06
099C-   20 61 4D    JSR   $4D61
099F-   A9 12       LDA   #$12
09A1-   85 05       STA   $05
09A3-   A9 71       LDA   #$71
09A5-   85 04       STA   $04
09A7-   A2 08       LDX   #$08
09A9-   20 98 64    JSR   $6498
09AC-   A2 08       LDX   #$08
09AE-   A0 04       LDY   #$04
09B0-   A9 0F       LDA   #$0F
09B2-   20 EA FE    JSR   $FEEA
09B5-   A9 00       LDA   #$00
09B7-   8D 80 12    STA   $1280
09BA-   A0 1F       LDY   #$1F
09BC-   B1 12       LDA   ($12),Y
09BE-   85 02       STA   $02
09C0-   C8          INY
09C1-   B1 12       LDA   ($12),Y
09C3-   85 03       STA   $03
09C5-   20 24 66    JSR   $6624
09C8-   F0 0B       BEQ   $09D5
09CA-   20 3C 4E    JSR   $4E3C
09CD-   A5 80       LDA   $80
09CF-   8D 0D F6    STA   $F60D
09D2-   4C C2 5E    JMP   $5EC2
09D5-   AD CE F6    LDA   $F6CE
09D8-   D0 03       BNE   $09DD
09DA-   4C 40 71    JMP   $7140
09DD-   A9 F6       LDA   #$F6
09DF-   85 03       STA   $03
09E1-   A9 CE       LDA   #$CE
09E3-   85 02       STA   $02
09E5-   A9 12       LDA   #$12
09E7-   85 15       STA   $15
09E9-   A9 99       LDA   #$99
09EB-   85 14       STA   $14
09ED-   A2 02       LDX   #$02
09EF-   A0 14       LDY   #$14
09F1-   A9 0C       LDA   #$0C
09F3-   20 63 4D    JSR   $4D63
09F6-   20 3F 72    JSR   $723F
09F9-   8A          TXA
09FA-   D0 CE       BNE   $09CA
09FC-   AD 21 12    LDA   $1221
09FF-   D0 00       BNE   $0A01
0A01-   00          BRK
