I have tested some UNROM Games to see how they function on IRQ execute BP :
Game : Castlevania (E) [!].nes
IRQ : BF C0
An execute BP on C0BF stops the game constantly
By changing BF C0 to BF 00 still it works fine
Code:
C0BF:40 RTI
C0C0:A5 FE LDA $00FE
C0C2:A6 1F LDX $001F
C0C4:F0 02 BEQ $C0C8
C0C6:A9 00 LDA #$00
C0C8:8D 01 20 STA $2001
C0CB:20 89 F8 JSR $F889
Game : Contra (U) [!].nes
IRQ : B5 C0
An execute BP on C0B5 stops the game constantly
By changing B5 C0 to B5 00 still it works fine
Code:
C0B5:40 RTI
C0B6:A5 FE LDA $00FE
C0B8:A6 20 LDX $0020
C0BA:F0 02 BEQ $C0BE
C0BC:A9 00 LDA #$00
C0BE:8D 01 20 STA $2001
C0C1:AD 00 80 LDA $8000
C0C4:48 PHA
C0C5:A5 1B LDA $001B
C0C7:30 08 BMI $C0D1
C0C9:A0 01 LDY #$01
C0CB:20 3F C1 JSR $C13F
Game : Duck Tales 2 (U) [!].nes
IRQ : 88 C0
RV : 88 C0
An execute BP on C088 DOES NOT stop the game!
By changing 88 C0 to 88 00 still it works fine!
Code:
C088:78 SEI
C089:A2 00 LDX #$00
C08B:8E 00 20 STX $2000
C08E:8E 01 20 STX $2001
C091:CA DEX
C092:9A TXS
C093:A2 02 LDX #$02
C095:2C 02 20 BIT $2002
C098:10 FB BPL $C095
C09A:2C 02 20 BIT $2002
C09D:30 FB BMI $C09A
Game : Megaman (U) [!].nes
IRQ : 08 C0
RV : 08 C0
An execute BP on C008 DOES NOT stop the game!
By changing 08 C0 to 08 00 still it works fine!
Code:
C008:78 SEI
C009:A9 01 LDA #$01
C00B:8D 00 20 STA $2000
C00E:A9 06 LDA #$06
C010:8D 01 20 STA $2001
C013:A9 05 LDA #$05
C015:8D 05 C0 STA $C005
C018:4C 00 90 JMP $9000
It seems that whenever IRQ is not needed, it is the same as Reset Vector, at least in these two games.
As a result I feel that I can use Duck Tales 2 or Megaman for the last bank of AT27C080 without worrying about its IRQ
After all, I don't want a AT27C080 go waste!
Thank you all for the help