Edit 1: Updated with krzysiobal's comment.
Edit 2: Updated with krzysiobal's final pinout, and analysis based on that.
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NTDEC8701 AX-24G
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Pin# Meaning Pin# Meaning
1 PRG-A13 1 ?
2 PRG-A14 2 ?
3 PRG-A16 3 ?
4 PRG-A15 4 ?
5 CHR-A14 5 to 15
6 CHR-A11 6 PRG-A15
7 CHR-A13 7 CHR-A14
8 CHR-A12 8 CHR-A13
9 CPU-D2 9 CPU-D0
10 CPU-D3 10 CPU-D1
11 CPU-D1 11 CPU-D2
12 GND 12 GND
13 CHR-A15 13 to 16
14 CHR-A10 14 ?
15 PPU-A10 15 to 5
16 PPU-A11 16 to 13
17 PPU-A12 17 Vcc
18 CPU-D4 18 CPU-D3
19 CPU-D5 19 CPU-D4
20 CPU-D0 20 CPU-D5
21 CPU-R/W 21 CPU-R/W
22 M2 22 M2
23 PRG-!OE 23 PRG-!OE
24 CPU-ROMSEL 24 CPU-ROMSEL
25 Vcc 25 ?
26 CPU-A0 26 CPU-A0
27 CPU-A13 27 CPU-A13
28 CPU-A14 28 CPU-A14
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Jovial Race Normal
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D5 D0
D2 D1
D0 D2
D1 D3
D3 D4
D4 D5
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Jovial Race Normal
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$01 $04
$04 $02
$21 $05
$24 $03
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$10 for 8 KiB CHR-ROM bank 0
$01 for 8 KiB CHR-ROM bank 1
$1A for 8 KiB CHR-ROM bank 2
$0B for 8 KiB CHR-ROM bank 3
OR'd with $04 to choose 32 KiB PRG-ROM bank 1.
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Jovial Race Normal
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$10 $20 for 8 KiB CHR-ROM bank 0
$01 $04 for 8 KiB CHR-ROM bank 1
$1A $38 for 8 KiB CHR-ROM bank 2
$0B $1C for 8 KiB CHR-ROM bank 3
OR'd with:
$04 $02 and for 32 KiB PRG-ROM bank 1
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$02 Normally CHR A11, pin 6, which is PRG A15 on Jovial Race.
$04 Normally CHR A12, pin 8, which is CHR A13 on Jovial Race.
$08 Normally CHR A13, pin 7, which is CHR A14 on Jovial Race.
$10 Normally CHR A14, pin 5, which Jovial Race connects to PPU-A10 (pin 15).
$20 Normally CHR A15, pin 13, which Jovial Race connects to PPU-A11 (pin 16).
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8 KiB
CHR-ROM $8001 IC CHR
bank bits bits range
------- ----- ----- -----
0 $10 $20 $1800-$1BFF
1 $00 $00 $1000-$13FF
2 $18 $30 $1C00-$1FFF
3 $08 $10 $1400-$17FF
(only bits relevant for the CHR range shown)
To summarize: Everything checks out that Jovial Race's AX-24G is a Namco-108-compatible NTDEC8701 connected to function like a simple GNROM-like latch. The reason for doing so must have been either that Joy Van had a surplus of these chips lying around in those early days, or as a (rather ineffective) copy-protection measure.
Thanks two million, krzysiobal and sdm! Comments and corrections welcome.