How can a Z80 assembly program find out its own memory address?

Question

I'm wondering how to write a program in Z80 assembler that discovers its own memory location.

I thought that maybe I could somehow load the program counter PC into, for example, BC.

Is there a way of doing this?

I'm messing with a single board Z80 system with 2 KiB ROM, 2 KiB RAM and that's about it. No operating system, just a very simple monitor program in the ROM that lets me inspect memory, load data to memory and connect a terminal to the board via a serial port. The assembler I'm using is z80asm on Linux.

Answer 1

If there are two consecutive bytes of RAM one can write at a known address, one could store the byte values E1h, E9h [POP HL / JP (HL)] at that address and then CALL it to place the address following the call into HL. Alternatively, if those byte values appear at some known address in ROM one could simply call that address likewise. There isn't any way to find out the address of executing code without being able to CALL some function at a known address.

Answer 2

In general:

1. Disable interrupts (and hope that NMI will not occur)

2. Perform "CALL" to any RET with known address (e.g. CALL $007C at ZX Spectrum, feel free to consult your system ROM listing, look for any $C9)

3. Lower stack one address (it should be the return address of the previous CALL): DEC SP, DEC SP

4. Get an address to a BC pair: POP BC

Answer 3

General answer that works for all processors:

• call a subroutine
• in this subroutine, get the value stored in the stack pointer (read (SP) as a 16 bit word on a Z80. 32 bit processors would require that your read a 32 bit word). This tells the return address, which is the address of the instruction coming after the subroutine call.
• now return this value in a register: you now have the value of the current program counter in this register. By doing some additions/subtractions, you can locate any program locations close to that point (be careful of assembler optimizations that may change the size of the instructions if you do that)

I'm far from being a z80 specialist (actually this code could be completely wrong), but something like:

   call  subr
   ; here af register contains the current PC location
   ; subtract 3 to get address before the "call" if it's of any use
   ...

subr:
    ld   af,(sp)
    ret

On a 68000 processor (I know it's not asked but there's more chance that the code is correct...) adapting Ross suggestion to avoid returning from a routine. Just pop the value from the stack to get PC and "forget" the subroutine call.

   bsr.w  subr  ; not possible to use .b with exact next address
subr:
   move.l   (A7)+,D0   ; pops the stack, get 32 bit value

For the record, on a 680x0 it's even simpler with PC relative load effective address instructions:

   lea addr(pc),a0   ; label value is in address register
addr:

Answer 4

In general case, when the environment is unknown, the answer is simply "it can't".

However, for a known environment, it is almost always possible.

Since your environment is unknown, I'll assume it is ZX Spectrum. For other environments, find a suitable suggestion.

1. If your program is called from BASIC, calling address is always in BC, as you wanted it (ZX Spectrum ROM specific).

2. Call the routine POP HL:JP [HL] at the known address, as already answered by @supercat. There are no such bytes in the original ZX Spectrum ROM, however.

3. Make your own POP HL:JP [HL] subroutine at the known free RAM address: LD HL,0xE9E1: LD [ADDR],HL:CALL ADDR. On ZX Spectrum, this could be video RAM or printer buffer (if run from BASIC).

4. If you can guarantee there would be no interrupts (no NMI or you can disable INTs by DI), simply call to the RET 'subroutine', then recover return address like this: DEC SP:DEC SP:POP HL. Again you can either call a known ROM address or organize a byte at the known free RAM area.

Answer 5

If you are in the ZX Spectrum, BC is loaded with the calling address given to USR in BASIC. So at the beginning of a C/M program it is guaranteed you know the address.

Also, we do not be want/cannot be doing CALLs to our own code if we are trying to find where it was loaded. (for instance, for doing relocation/using relocation CALL tables as the GENS3 assembler did back in the day to be able to be loaded in ANY RAM position).

Also doing CALLs to ROM, does not guarantee you will have your address in the stack by the time you are trying to fetch it, as an interrupt can happen in the meanwhile, unless you got your interrupts disabled.

We can "cheat". For instance, with interrupts enabled, and knowing we only have interrupts 1/50 of a second, so we dont have to disable interrupts after the HALT for preserving the stack:

ORG 50000
LD  HL,0
ADD HL,SP
DEC HL
DEC HL
HALT
LD  C,(HL)
INC HL
LD  B,(HL)
RET

After the halt, we know a interrupt has happened, and (SP-2) will still have the address of the instruction after the HALT - LD C,(HL)

So in the ZX Spectrum:

PRINT USR 50000 will return 50007.

PS. Following @Raffzahn tips for code optimization:

ORG 50000
LD    HL,$FFFE
ADD   HL,SP
HALT
LD    SP,HL
POP   BC
RET 

PRINT USR 50000

returns 50005 (the address of SP,HL after HALT) -- e.g. BC is loaded with the address of the instruction after HALT.

PPS in a simple board, the same can be accomplished via:

ORG   XXXX
LD    HL,$FFFE
ADD   HL,SP
DI
CALL  ADDRESS_IN_MONITOR_ROM_OF_A_RET_OPCODE
LD    SP,HL
POP   BC
EI
RET 

Answer 6

I'm wondering how to write a program in assembler that discovers it's own memory location.

This depends much on the CPU used, as well as the OS/computer. Most important here if the code executed is relocated when loaded (*1) or not. The later is relevant for ROM code that may get inserted at different addresses.

I/O drivers (slot PROM) of Apple II expansion cards are a great example here. Each slot has an assigned address range for a driver ROM as well as I/O registers. To work accordingly the ROM code had to find it's own address first. Since a ROM isn't relocated it couldn't simply call an address within. Instead a 'well known RTS' (Return instruction) in Monitor ROM was called. After returning, the return address could be peeked from stack, much like this:

STACK    .EQ   $100
KNOWNRTS .EQ   $FF58   ;KNOWN RTS INSTRUCTION (*1)

         JSR   KNOWNRTS
         TSX
         LDY   STACK,X     ;PCH
         LDA   STACK-1,X   ;PCL

This is complete independent form any code relocation, but only works if there is a known Return instruction. Under some OS this may as well be possible, like the MS-DOS PSP containing a RETF at location 52h (*2).

Another way is to call some (informative) OS function and later on examine the stack.

Now, if the code is relocated when loaded, a Subroutine Call can be used to emulate pushing the address:

         JSR   NEXT
NEXT:    PLY   ;PCH
         PLA   ;PCL

Of course if relocation is done by anything but the most simple relocator, it will as well adjust address constants, so it simply can be defined as memory word:

THIS     DW    THIS

I thought that maybe I could somehow load the program counter into for example BC.

This sounds as if you imply an 8080 type CPU. The principles are the same and the examples may look like this:

Using a known return:

KNOWNRET EQU   ???      ;Known Return, depends on OS/machine

         CALL  KNOWNRET
         DEC   SP
         DEC   SP
         POP   BC

Code relocated when loaded (really short now):

         CALL  NEXT
NEXT:    POP   BC       ;POP B in 8080 syntax

Now BC contains the address of NEXT.

Or even shorter by letting Assembler/linker do the work:

THIS     LD    BC,THIS  ;LXI B,THIS in 8080 syntax

Bottom line, it all comes down to the environment and how much can be used.

---

P.S.: For all of these examples disabling interrupts may be needed, again depending on CPU, OS and environment.

---

*1 - That is, the loader patches all absolute references in a piece of code according to the location the code is loaded.

*2 - $FF58 is part of the Restore subroutine of the Apple II Monitor ROM and used wherever a Return or null-function is needed. For example does BASIC initialize the '&' extension vector ($3F5) with a jump to $FF58.

*3 - Then again in x86 everything is segment based and the actual segments can be read out anyway.

Answer 7

The Z-80 has no direct way to read the program counter contents. However, there are a few techniques that can work with minimal assumptions. If your code is loaded into RAM it can use self-modification to find itself:

        di
        ld      hl,0
        ld      a,(hl)
look:   ld      (hl),a
        dec     hl
        ld      a,(hl)
        ld      (hl),0
loc:    jr      look
        ld      (hl),a      ; repair, HL = loc + 1

When this completes HL will point to loc + 1. The program marches down from the top of memory temporarily setting each memory location to 0. When it gets to loc + 1 the jr there will be changed to go to the next instruction. At which point the jr is fixed and the program now knows where it is in memory.

If your program is in a ROM at an unknown location you could do something more complicated in terms of checksumming memory in order to identify your code. Or look for some random "signature" bytes. Or if you know there are two bytes of RAM at a certain memory location you can do this:

        ld      de,($3c00)
        ld      hl,$e9e1
        ld      ($3c00),hl  ; write $e1 $e9 which is POP HL; JP (HL)
        call    $3c00
loc:    ld      ($3c00),de  ; HL = loc

If you only know one byte of RAM, this will do:

        di
find:   ld      hl,mem
        ld      a,(hl)
        ld      (hl),$c9    ; "RET"
        call    mem
loc:    ld      (hl),a      ; restore
        dec     sp
        dec     sp
        pop     hl          ; now HL==loc

Though it does have to be careful that the RAM modified does not happen to be one of the two bytes of the stack used. There are ways around this but for most systems you're highly likely to be able to pick a RAM location that does not conflict.

The DI instruction stops most interrupts which would change the stack address if one occurs after the ret is executed. However, the non-maskable interrupt (NMI) can't be stopped. One workaround is to validate the address loaded into HL say by ensuring it points to a ld (hl),a instruction. Add this code:

        ld      a,(hl)
        cp      $77     ; LD (HL),A opcode
        jr      nz,find

Obviously that could be fooled by an unlucky data being placed on the stack. Further checks can be made to reduce the chances of such a coincidence to practical certainty.

Or you could make the almost perfectly safe assumption that an NMI will merely change the top of the stack to an address within your program. In that case you only need to search back for the ld (hl),a instruction to lock down the position:

        di
        ld      hl,mem
        ld      a,(hl)
        ld      (hl),$c9    ; "RET"
        call    mem
loc:    ld      (hl),a      ; restore
        ld      hl,0
        add     hl,sp
        dec     hl
        ld      a,(hl)
        dec     hl
        ld      l,(hl)
        ld      h,a
look:   ld      a,(hl)
        cp      $77         ; LD (HL),A opcode
        dec     hl
        jr      nz,look
                                ; now HL==loc - 1

These solutions may not be entirely practical but they do point out how to work with minimal system knowledge.

Answer 8

The Nascom II system ROM NAS-SYS 3 reserved one of the RST instructions (RST 10h) for emulating a relative call. On that computer, obviously

RCAL $+2
POP HL

would do the trick for getting the current address (+2) into HL. The 1k debugger extension ROM used this and other tricks in order to be fully relocatable. Of course, this is not a generally useful answer since it is system dependent. But it was a nice byte-saving feature which NAS-SYS itself used quite a bit.

Answer 9

To close the circle on some of the other answers:

Amongst other features, the Z80 supports CALL only to absolute addresses. So, either:

(i) you can't use CALL at all in your hypothetical system (or therefore any internal subroutines); or (ii) you can assume a runtime loader modified the target of your CALLs.

Therefore, the knee-jerk answer is don't even bother using the stack as others have suggested. Just directly read the argument of a CALL, assuming your runtime loader is smart enough to modify absolute loads. There's no PC-relative loading so that's likely. If so then:

programStart:
    ... lots of program code here ...

    LD HL, (dummycall+1)
    JR dontreally

dummycall:
    CALL programStart
dontreally:

EDIT: or, as Raffzahn points out, if you're assuming a loader that has fixed your absolute addresses then there's no need for the artificial complexity:

programStart:
    LD HL, programStart

If you can't assume that your absolute addresses have been fixed, but can assume a working stack, you can do an artificial CALL and pop as otherwise suggested.

If you've been asked to write code for a Z80 in which you may have been arbitrarily relocated, but your internal addresses haven't been adjusted then:

• you can't use CALL; and
• can't use any version of LD except immediate.

So you're in the realm of the vaguely ridiculous.

Even leaking the PC via MEMPTR requires you to be able to make assumptions about both the memory map (as MEMPTR leaks via BIT n, (HL) only, so you'll need to be able to do this with a suitable HL) and the IO map (as you'll need CPI and CPD to get the full contents of the leaked PC given that only bits 3 and 5 leak). So that's out.

So, in net:

• either your linker will patch up your CALL addresses and the above snippet will work — and other than for academic purposes, it's difficult to imagine a scenario in which that won't be true;
• or it won't in which case if you know nothing whatsoever about the rest of the memory map then I currently think the task is impossible;
• but if you at least know two contiguous IO addresses and a single memory address where reading has no consequence, you can use a CPIR/CPDR that executes at least twice to load the PC into MEMPTR, then use BIT n, (HL), CPI and CPD very slowly to inspect it.

Answer 10

One method that hasn't been mentioned is writing to a safe memory buffer with the pop hl, jp (hl) call. For example if I know there is space at $4000 (ZX Spectrum screen memory) but I don't where my program has been loaded I can:

ld hl,$e9e1    ; pop hl jp (hl) combination
ld ($4000),hl
call $4000

; hl will now be the loaded address of this next instruction
...

Answer 11

Platform independent:

• scan the memory for a RET opcode.
• call that memory location
• adjust the stack pointer
• pop from the stack to get the address of your call

Answer 12

Since we now know that the OP uses a DIY Z80 system and z80asm for generating its machine code, I feel the need to add this simple answer:

Use the special expression $:

The special value "$" is the address of the first byte of the current command.

With:

    ld hl,$

you will get the address of the opcode of ld hl,imm16 in HL.

Or:

    dw  $

will place the address of the first byte of the stored address at this place.

Expanding this leads to this additional simple solution:

start:
    ; any code you like
    ld hl,start
    ; ...
    dw start

Let the assembler do the work. Developers are lazy, aren't you?