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PostPosted: Fri Jun 04, 2021 6:21 pm 
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Joined: Fri Jun 04, 2021 5:45 pm
Posts: 5
Location: Chelmsford, Essex
Hi,

Newbie question here!

So I want to store a value at memory location $5940.

I have another routine which calculates this memory location and it stores it in zero page as this

Location $70 = $40
Location $71 = $59

So I thought I could indirectly store to this location using:

LDA value to be stored
LDX #0
STA (&70),X

But I can't get this to work, I'd appreciate some advise.

Thanks.


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PostPosted: Fri Jun 04, 2021 6:51 pm 
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Joined: Fri Aug 30, 2002 1:09 am
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Welcome.

There's (ZP,X) addressing, and there's (ZP),Y addressing, but no (ZP),X addressing. The 65c02 also has (ZP) addressing (with no indexing), so you don't need X or Y for this.

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PostPosted: Fri Jun 04, 2021 7:35 pm 
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Joined: Fri Jun 04, 2021 5:45 pm
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Location: Chelmsford, Essex
GARTHWILSON wrote:
Welcome.

There's (ZP,X) addressing, and there's (ZP),Y addressing, but no (ZP),X addressing. The 65c02 also has (ZP) addressing (with no indexing), so you don't need X or Y for this.


Thanks for your response, I found this explanation too:

INDIRECT INDEXED ADDRESSING - In indirect indexed addressing (referred to as ( Indirect) , Y), the second byte of the instruction points to a memory location in page zero.
The contents of this memory location is added to the contents of the Y index register, the result being the low order eight bits of the effective address.


So this should work:

Location $70 = $40
Location $71 = $59

LDA value to be stored
LDY #0
STA (&70),Y

Which should store data in $5940 (made up from $59$40 with zero offset)

Is that correct?


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PostPosted: Fri Jun 04, 2021 8:30 pm 
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Joined: Thu May 28, 2009 9:46 pm
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Location: Midwestern USA
archie456 wrote:
Hi,

Newbie question here!

So I want to store a value at memory location $5940.

I have another routine which calculates this memory location and it stores it in zero page as this

Location $70 = $40
Location $71 = $59

So I thought I could indirectly store to this location using:

LDA value to be stored
LDX #0
STA (&70),X

But I can't get this to work, I'd appreciate some advise.

Thanks.

As Garth noted, (<zp>),X addressing doesn't exist. In a syntactically-correct assembler, the form of your code would depend upon whether writing for an NMOS 6502 or a 65C02. In the former case, you would write:

Code:
          LDY #0
          STA ($70),Y

With the 65C02, you can shorten the above to:

Code:
          STA ($70)

Note that in 6502 assembly language, a dollar sign ($) is used to indicate hexadecimal. This is a convention that was inherited from Motorola (the designers of the 6502 were ex-Motorola engineers). Use of the ampersand (&) is ambiguous at best—it's often interpreted as a Boolean symbol for logical AND.

Surrounding the page-zero address with parentheses tells the assembler to use indirection.

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PostPosted: Fri Jun 04, 2021 9:08 pm 
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Joined: Fri Jun 04, 2021 5:45 pm
Posts: 5
Location: Chelmsford, Essex
Perfect! Thanks for your responses.

(its a 6502 I'm programming for... Also a BBC Micro which uses & for hex...)

Cheers.


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