Assembly

[wilsonsamm]

Hello there everyone!
I'm just beginning to dabble along with 68k assembler. I've been doing it on and off since friday and I've come up with a simple sumpink for emulating the CPU of the PDP-8. Loads of things are still missing, but first of all I want to get the routine for indirect adressing working properly.

So, here's my source. first comes the pdp-8 code, explained below, and then the "emulator" starts at org $1000.
A0 I've designated to the emulated machine's program counter.
A1 holds the adress to the operand which the emulated machine current working on.
D0 holds the opcode which is being executed.
D1 is the accumulator of the emulated machine.

Anyway, here comes the pdp-8 code I'm trying to run, with addresses and machine code in octal

0000 1007 tad   0007  ;add to AC whatever's in location 7
0001 3406 dca i 0006   ;deposit AC to the location pointed to by 0006, and clear AC.
0002 2006 isz   0006   ;increment location 0006 by one, and skip next instruction if 0
0003 5000 jmp   0000   ;jump to location 0000
0004 7402 hlt          ;halt processor.
0005 
0006 0010
0007 5252
0010 ; first location to be written to

and here is the "emulator"

* pppp     d pppp   888     / ee   mm mm  
* p   p    d p   p 8   8   / e  e m  m  m 
* pppp  dddd pppp   888   /  eee  m  m  m 
* p    d   d p     8   8 /   e    m  m  m 
* p     dddd p      888 /     eee m  m  m 
*
* by  s a m.  f r.  j u l.  2 0.  2 0 0 7.
* version zero. attempts to run pdp-8 code.

memloc0	dc.w	$0207		* tad   0007
memloc1	dc.w	$0706		* dca i 0006
memloc2	dc.w	$0406		* isz   0006
memloc3	dc.w	$0a00		* jmp   0000
memloc4	dc.w	$0F02		* hlt
memloc5	dc.w	$0000
memloc6	dc.w	$0008		* starting adress
memloc7	dc.w	$0aaa		* fill pattern
START	org	$1000
OPFETCH	move.w	(a0)+,d0		put current opcode into d0.
	move	d0,d2		time to calculate what adress is being reffed to
	andi.w	#$FF,d2
	asl	#$1,d2		make sure we've got an even adress.
	bclr	#$8,d2		zero page?
	beq	DIRECTION		yup. that's what we have now, so see if we want indirection.
	move	a0,d3		otherwise, get page from upper 5 bits of PC.
	ori.w	#$FF80,d3
	or.w	d3,d2
DIRECTION	btst	#$9,d0		now, are we dealing with indirect adressing?
	beq	INDIRECT		yes, so do it.
	move	d2,a1		otherwise, put address into appropriate register
	jmp	OPPARSE		and see what's next to go down inda hood.
INDIRECT	move.w	d2,a2
	move.w	(a2),a1		use a2 as scratchpad for loading effective adress into adress register
OPPARSE	asr	#$8,d0		get rid of everything but the command.
	asr	#$1,d0		we have everything we need.
	cmpi	#$0,d0
	beq	ANDOP
	cmpi	#$1,d0
	beq	TADOP
	cmpi	#$2,d0
	beq	ISZOP
	cmpi	#$3,d0
	beq	DCAOP
	cmpi	#$4,d0
	beq	JMSOP
	cmpi	#$5,d0
	beq	JMPOP
	cmpi	#$6,d0
	beq	IOTOP
	cmpi	#$7,d0
	beq	OPROP
ANDOP	move	(a1),d2		AND - self explanatory
	and	d2,d1
	jmp	INTER
TADOP	add	(a1),d1		TAD - add operand to accumulator
	jmp	INTER
ISZOP	move.w	(a1),d2		ISZ - increment operand by 1. Skip next instruction if it overflows to 0
	addq.w	#$1,d2
	andi.w	#$0fff,d2
	bne	NOSKIP
	addq	#$2,a0
NOSKIP	andi.w	#$f000,(a1)		rightmost twelve bits will always be 0 in this case
	or.w	d2,(a1)
	jmp	INTER
DCAOP	move.w	d1,(a1)		DCA - deposit and clear accumulator
	move.w	#$0,d1
	jmp	INTER
JMSOP	move	a0,(a1)+		JMS - store PC at word
	move	a1,a0		and continue execution from the following word
	addq	#$2,a0		execute next command after that.
	jmp	OPFETCH		in this case, don't allow an interrupt to be dealt with till next round
JMPOP	move.w	a1,a0		JMP - jump
	jmp	INTER
IOTOP	addq	#$2,a0
	jmp	INTER		IOT - input output transfers. fill this in later
OPROP	addq	#$2,a0		OPR - microcoded operations. fill this in later.
	jmp	INTER
INTER	nop			deal with interrupts. fill this in later
	jmp	opfetch		

	MOVE.B	#9,D0
	TRAP	#15		Halt Simulator

	END	START

the first few pdp-8 instruction run just fine. but then something goes tits up in the routine that deals with indirect adressing, which has unwanted consequences on the rest of the program, because the third instruction then doesn't work.

Can anyone here tell what's up with my code?

[Harrison]

Sorry, I never bothered learning assembler, instead sticking to higher level languages. Hopefully others here might be able to help. Submeg?

[Puni/Void]

Sadly I can't help you with your queries either, but hopefully someone else can be of assistance. Cool to see that you are learning Assembler by the way! Good luck with the source!

[wilsonsamm]

Thanks! It's cool to be learning it! It's always good to learn something new.
Maybe there's another 68k board somewhere on the net someone could suggest to me?

[Teho]

You should definitely try one of the demoscene boards like the one on scene.org then. If anyone knows 68k assembler, it's them.

[Submeg]

Yea I will have a look later, but I havent done it in a while so it may take a little while

[wilsonsamm]

I sussed it
When the adress is direct, there's a command which says "asl.w #$1,d2". this basically doubles the adress, so that this number is the adress in bytes, like the 68k does it, not words as does the PDP-8. This of course, has to be done again if it's indirect adressing aswell.
I'm well chuffed

[Submeg]

nice work man...I was like lol