Code:
ROM:198D ; =============== S U B R O U T I N E =======================================
ROM:198D
ROM:198D
ROM:198D sub_198D: ; CODE XREF: ROM:1061p
ROM:198D ; sub_1343+17p ...
ROM:198D ld h, 1 ; 100h
ROM:198F ld a, b ; a = b - c
ROM:1990 sub c
ROM:1991 jr nc, loc_199E ; if a xxx 0 goto loc
ROM:1993 neg ; a = -a
ROM:1995 srl a ; a >>= 1 (a /= 2)
ROM:1997 ld c, a ; c = a
ROM:1998 add a, b ; a += b
ROM:1999 ld l, a ; a = (100h)[a]
ROM:199A ld a, (hl)
ROM:199B ld l, c ; a -= (100h)[c]
ROM:199C sub (hl) ; alternatively, swap b and c, then do the below
ROM:199D ret
ROM:199E ; ---------------------------------------------------------------------------
ROM:199E
ROM:199E loc_199E: ; CODE XREF: sub_198D+4j
ROM:199E srl a ; a >>= 1 (a /= 2)
ROM:19A0 ld b, a ; b = a
ROM:19A1 add a, c ; a += c
ROM:19A2 ld l, a ; a = (100h)[a]
ROM:19A3 ld a, (hl)
ROM:19A4 ld l, b ; a -= (100h)[b]
ROM:19A5 sub (hl)
ROM:19A6 ret
ROM:19A6 ; End of function sub_198D
Data at 100h:
ROM:0100 db 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1
ROM:0100 db 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4
ROM:0100 db 5, 5, 5, 5, 6, 6, 6, 7, 7, 7, 8, 8, 9, 9, 9, 0Ah, 0Ah
ROM:0100 db 0Ah, 0Bh, 0Bh, 0Ch, 0Ch, 0Dh, 0Dh, 0Eh, 0Eh, 0Fh, 0Fh
ROM:0100 db 10h, 10h, 11h, 11h, 12h, 12h, 13h, 13h, 14h, 14h, 15h
ROM:0100 db 15h, 16h, 17h, 17h, 18h, 19h, 19h, 1Ah, 1Ah, 1Bh, 1Ch
ROM:0100 db 1Ch, 1Dh, 1Eh, 1Eh, 1Fh, 20h, 21h, 21h, 22h, 23h, 24h
ROM:0100 db 24h, 25h, 26h, 27h, 27h, 28h, 29h, 2Ah, 2Bh, 2Bh, 2Ch
ROM:0100 db 2Dh, 2Eh, 2Fh, 30h, 31h, 31h, 32h, 33h, 34h, 35h, 36h
ROM:0100 db 37h, 38h, 39h, 3Ah, 3Bh, 3Ch, 3Dh, 3Eh, 3Fh, 40h, 41h
ROM:0100 db 42h, 43h, 44h, 45h, 46h, 47h, 48h, 49h, 4Ah, 4Bh, 4Ch
ROM:0100 db 4Dh, 4Eh, 4Fh, 51h, 52h, 53h, 54h, 55h, 56h, 57h, 59h
ROM:0100 db 5Ah, 5Bh, 5Ch, 5Dh, 5Fh, 60h, 61h, 62h, 64h, 65h, 66h
ROM:0100 db 67h, 69h, 6Ah, 6Bh, 6Ch, 6Eh, 6Fh, 70h, 72h, 73h, 74h
ROM:0100 db 76h, 77h, 79h, 7Ah, 7Bh, 7Dh, 7Eh, 7Fh, 81h, 82h, 84h
ROM:0100 db 85h, 87h, 88h, 8Ah, 8Bh, 8Dh, 8Eh, 90h, 91h, 93h, 94h
ROM:0100 db 96h, 97h, 99h, 9Ah, 9Ch, 9Dh, 9Fh, 0A0h, 0A2h, 0A4h
ROM:0100 db 0A5h, 0A7h, 0A9h, 0AAh, 0ACh, 0ADh, 0AFh, 0B1h, 0B2h
ROM:0100 db 0B4h, 0B6h, 0B7h, 0B9h, 0BBh, 0BDh, 0BEh, 0C0h, 0C2h
ROM:0100 db 0C4h, 0C5h, 0C7h, 0C9h, 0CBh, 0CCh, 0CEh, 0D0h, 0D2h
ROM:0100 db 0D4h, 0D5h, 0D7h, 0D9h, 0DBh, 0DDh, 0DFh, 0E1h, 0E2h
ROM:0100 db 0E4h, 0E6h, 0E8h, 0EAh, 0ECh, 0EEh, 0F0h, 0F2h, 0F4h
ROM:0100 db 0F6h, 0F8h, 0FAh, 0FCh, 0FEh
The given function takes b and c as input and produces a as output.
This is Time Trax for Genesis's sound driver, and at this point in the program I come across a structure like
db command_byte
db unknown_parameter
dw jump_table_target_1
dw jump_table_target_2
dw jump_table_target_3
...
jump_table_target_1:
db command_bytes...
unknown_parameter is loaded into b and some other value is loaded into c; the result is the index into the jump table.
I am trying to determine one of the following two things:
1) Is unknown_parameter the size of the jump table? Or in other words, is b the upper bound of a?
2) What does this function do? I have no idea...
Thanks. |
