Calculate offset using labels

[Beska]

First off let me say I have no idea what I am doing. I'm just starting to figure out 68k assembly. Now that thats out of the way... I'm working on some code that as part of its operation, relocates itself and then continues on using an offset address. This code was originally written for Macro68 but I'm using DevPac 3.
The part where I'm struggling is where the program relocates itself and then jumps to the new address which is calculated as an offset of the new memory block.

The code in question looks like:

; a3 is a pointer to the newly allocated block of memory
donewithreloc:

jmp (newaddress,a3) ;execute from now on from the allocated

;memory space

newaddress:
;Allocate four 220K chunks of memory for the big buffer

moveq #$16,d4
rol.l #8,d4 ;512*11 trackdisk buffer
lea (RamPtr+FastBoot,pc),a4
movea.l a4,a3

When this is assembled in DevPac I get an error "Relative not allowed" at the jmp.

So my question is, Is there another way to calculate this offset in DevPac without getting the "Relative not allowed" error? I am pretty sure there must be a way to do this but since I'm not experienced in 68k assembly I have to turn to more knowledgable folk.

[Galahad/FLT]

Originally Posted by Beska

First off let me say I have no idea what I am doing. I'm just starting to figure out 68k assembly. Now that thats out of the way... I'm working on some code that as part of its operation, relocates itself and then continues on using an offset address. This code was originally written for Macro68 but I'm using DevPac 3.
The part where I'm struggling is where the program relocates itself and then jumps to the new address which is calculated as an offset of the new memory block.

The code in question looks like:

; a3 is a pointer to the newly allocated block of memory
donewithreloc:

jmp (newaddress,a3) ;execute from now on from the allocated

;memory space

newaddress:
;Allocate four 220K chunks of memory for the big buffer

moveq #$16,d4
rol.l #8,d4 ;512*11 trackdisk buffer
lea (RamPtr+FastBoot,pc),a4
movea.l a4,a3

When this is assembled in DevPac I get an error "Relative not allowed" at the jmp.

So my question is, Is there another way to calculate this offset in DevPac without getting the "Relative not allowed" error? I am pretty sure there must be a way to do this but since I'm not experienced in 68k assembly I have to turn to more knowledgable folk.

When using PC relative code, 220k offset is too big, max size for PC relative offsetting is $7fff

[Beska]

The offset will be significantly smaller than 220k, just enough to jump over the initial copy code.

[Don_Adan]

Originally Posted by Beska

First off let me say I have no idea what I am doing. I'm just starting to figure out 68k assembly. Now that thats out of the way... I'm working on some code that as part of its operation, relocates itself and then continues on using an offset address. This code was originally written for Macro68 but I'm using DevPac 3.
The part where I'm struggling is where the program relocates itself and then jumps to the new address which is calculated as an offset of the new memory block.

The code in question looks like:

; a3 is a pointer to the newly allocated block of memory
donewithreloc:

jmp (newaddress,a3) ;execute from now on from the allocated

;memory space

newaddress:
;Allocate four 220K chunks of memory for the big buffer

moveq #$16,d4
rol.l #8,d4 ;512*11 trackdisk buffer
lea (RamPtr+FastBoot,pc),a4
movea.l a4,a3

When this is assembled in DevPac I get an error "Relative not allowed" at the jmp.

So my question is, Is there another way to calculate this offset in DevPac without getting the "Relative not allowed" error? I am pretty sure there must be a way to do this but since I'm not experienced in 68k assembly I have to turn to more knowledgable folk.

Use:
lea (RamPtr+FastBoot),a4

[phx]

It's not surprising that Devpac doesn't like the "jmp (newaddress,a3)".
"newaddress" is a label, so it cannot be used as 16-bit displacement on A3.

But I'm not sure if I, or anybody else here, understood what you are trying to do. Am I correct that you want to relocate the code below "newaddress", and you already copied it to a new memory region with its base address stored in A3? Then you only have to jump to this address: "jmp (a3)". :

[Beska]

yes, newaddress is where I want to jump to at a3. The code starting at a3 will have the relocation code. Once that has executed I don't need to execute it again so I want to skip over it.

[Leffmann]

You have to compute the offset with

jmp (start-orig, a3)

, but I would just do as phx suggested and not move the relocation code at all.

[Beska]

Your suggestion seems to do the trick. I can't believe I didn't think of this myself.
thanks!