How do I perform a long jump to a numerical address from my C code? I'm pretty sure I have to do this with inline assembly - however that seems complicated for C51. I figured in C I could do this:
typedef void (*jmpPtr)(void); jmpPtr jmp; ... unsigned int address = 0x4000U; jmp = (jmpPtr) address; (*jmp)();
Gee, and just when things were looking easy... Well, I can give it a shot. It obviously depends on whether the straight-forward version already posted will work first. Since you are probably following all of this thread, you know that Mark says it won't work, and since I haven't tried it yet, I can't say whether he's right. But, in case it does work, can you provide us with some more information? I take it that you'll be performing this "jump" from within an ISR, right? Upon entry, ISR has pushed some processor context on the stack and possibly even switched register banks. What kind of "fixing up" is required? We need to know what, if any (and hopefully none), of the saved processor context needs to be preserved across the jump. Are you jumping from interrupt context to interrupt or non-interrupt context? Or is it more like a software reset to a dynamic address? You're still wanting all this done in C, right? --Dan Henry
Actually, I was trying to do all this in C so I wouldn't have to figure out how to do 'inline' assembly from C. However, it may just be easier to do the assembly. What I am doing is caputring an interrupt and passing it on to another interrupt handler. So it would be interrupt handler to interrupt handler. Now that I think about it I suppose I would have to fix up the stack regardless, otherwise I'll crash... Any help would be appreciated of course.
Your needing an ISR-to-ISR jump complicates things a bit. Conceptually, the "jump using RET" is still do-able, but a C interrupt function returns using RETI instruction to restore the interrupt priority logic and that's (probably) not desireable at that point in servicing the interrupt. There's also the issue of differences in saved processor context needs between ISR functions. Maybe I've fallen victim to some form of limited thinking, but to reliably do what you want requires a bit of assembler, I think. Keil doesn't support inline assembly in the way we both probably understand the term, so I'm providing a short assembly language excerpt to show how I'd dynamically re-vector an interrupt. The idea is to provide a fixed ISR front-end that will "jump" through an address you provide in a global. That address is the dynamic address of your C interrupt function, which has presumably been relocated in flash. Since you want some ISR front-end code, you have to disable the compiler's automatic vector generation by using the NOINTVECTOR compiler option and write your own interrupt vectoring code. The following is a snippet for that assembler file (say, intvects.a51). I have tested this code and know it to work.
EXTRN DATA (Ext0_Addr) EXTRN DATA (Tmr0_Addr) CSEG AT 00003H Ext0_IntVector: ;** Calling the revector function pushes a ;* return address that will be overwritten ;* by the target ISR's address. ;** LCALL Ext0_Revector ; Never returning to here! CSEG AT 0000BH Timer0_IntVector: LCALL Timer0_Revector ; Never returning to here! ;** ==MORE VECTOR CODE SNIPPED== Ext0_Revector: USING 0 PUSH AR0 ; Save MOV R0,SP ; &Saved_R0 DEC R0 ; &RET_ADDR_MSB MOV @R0,Ext0_Addr ; Target ISR's addr MSB DEC R0 ; &RET_ADDR_LSB MOV @R0,Ext0_Addr+1 ; Target ISR's addr LSB POP AR0 ; Restore RET ; "Jump" to target ISR ; ..and RETI from there. Timer0_Revector: USING 0 PUSH AR0 ; Save MOV R0,SP ; &Saved_R0 DEC R0 ; &RET_ADDR_MSB MOV @R0,Tmr0_Addr ; Target ISR's addr MSB DEC R0 ; &RET_ADDR_LSB MOV @R0,Tmr0_Addr+1 ; Target ISR's addr LSB POP AR0 ; Restore RET ; "Jump" to target ISR ; ..and RETI from there. ;** ==MORE RE-VECTOR CODE SNIPPED== END