RORG and physicial ORG of code htat must be copied to ram (DASM)

[Windless]

Hi,

 

  When you want to modify code at runtime, the code has to be in RAM. Since RAM is random at switch-on, you need to copy the code from ROM to RAM and then modify it. But its address changes, that's where the RORG instruction comes handy : you can tell the assembler at which place the code will be in memory, so it can use the runtime, not compilation time, addresses. E.G. :

	RORG	$80		; RORGed address, tells the assembly "compile this code here, but do as if if were as $80"
start:
	nop			; this might physically be at e.g. $1500
	jmp		.loop	; will compile a jump to $80, no $1500, so this code is ready to be copied in RAM at $80
end:

 This works fine. You then make a loop with (end-start) iterations to copy this code from $1500 to $80. (end-start) here will be ($84-$80=)$04, and for destination address of course you can use the start label ($80). But for source destination, you can't use start which as been RORGed, so I usually do something like this :

physical_start:			; used as source address for the copy to RAM
	RORG	$80
start:
	...			; your code goes here
end:				; (end-start+1) is the code length

 so I can now use physical_start as a source address since it has not been RORGed

 

I was wondering if this is usually how people do it, if there are other ways, if some assemblers has a way of getting the physical address for a label (I could not find one in DASM documentation)...

Edited September 7, 2022 by Windless

[+Bruce-Robert Pocock]

Ref: "some assemblers"

 

64tass (Turbo Assembler) you'd do basically the same thing as what you wrote:

 

PhysicalAddress:
          .logical $02
DestinationAddress:

Loop:
          jmp Loop
          .here

where PhysicalAddress might be $f500 and DestinationAddress would be $0002, and jmp Loop would be jmp $0002