Hello,
I hope you will understand my post since English isn't my native language.
I am writing a NES emulator for a Motorola 68000 platform (a TI-89 calculator to be more precise). But I'm not very familiar with 6502 assembly, although I've read a few tutorials. The emu core is under development right now, and I was wondering if the 'absolute y' addressing mode is a signed displacement?
For example: sta $700,y with y containing #$fe.
In this case, will the content of A reg be stored at address $6fe ($700-2) or $7fe ($700+$fe)?
I would appreciate if someone could give me more details about all the addressing mode wich are ambigous in this sense.
Thanks by advance.

No, it is not signed. It goes to 7FE. Indexing is never signed, although ZP indexed will wrap around to remain in ZP. For example, FE in ZP, indexed by 3, will result in 01, not 101. If you wanted the 101, you'd have to force the assembler to use abs instead of ZP addressing, so 00FE + 3 = 101.

In absolute indexed instructions the index value is always unsigned so in your case an LDA $0700,X will load a byte from an address between $0700 and $07FF inclusive.

In zero page indexed instructions any carry from the low byte of the address caused by adding the index is ignored (because the processor forces the high byte of the address to be $00) so if X is $FF then LDA $80,X loads from $007F and not $017F. So these look signed.

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Andrew Jacobs
6502 & PIC Stuff - http://www.obelisk.me.uk/
Cross-Platform 6502/65C02/65816 Macro Assembler - http://www.obelisk.me.uk/dev65/
Open Source Projects - https://github.com/andrew-jacobs

I found a 6502 simulator on this site wich is a great help with all the doubts I had with addressing modes. Thanks