After I wrote that screed, I dug out the assembler and things and had a
look. This is what worked like strlen():-

.model small
.code
pointer dd string
string  db 'this is a string',0

start:  push es
        pushf   : preserve msw
        push di
        cld     ; go up
        les di,dword ptr pointer
        xor ax,ax
        mov cx,ax
        dec cx  ; set cx nonzero
        repne scasb
        inc cx  ; compensate for earlier dec cx
        sub ax,cx
        dec ax  ; discard '\0'
        pop di
        popf    ; reclaim msw
        pop es
        ret
        end

    This returns the length of the string in AX.

    Obviously it would need a C interface around it to be useful, but I
just wanted to confirm that I hadn't got it completely arse about face.

Best Wishes,
Bill.

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