4 Feb
2008
4 Feb
'08
4:14 p.m.
Subject: Re: "CP/M compatible" vs. "MS-DOS Compatible" machines?
From: "Dave Dunfield" <dave06a at dunfield.com>
Date: Mon, 04 Feb 2008 07:21:16 -0500
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>
I don't think he was talking about pushing data on the stack,
I thought he was refering to putting the system service request
number inline with the code.
The way you can "force that" in cpm is use the above code to call
say RST2 where the call is broken out and organized in registers
per cpm normal. It also has to preserve registers and adjust the
returned data to be on the stack but it's not that bad.
I'd
like to implement something that'd not only use one of those single-byte
calls, but take the data inline, as well -- no need to load it into
registers to pass it along, though there are some rather clumsy aspects to
getting the stack pointer and fixing it...
That was easy in 8080.
;routine gets 16bit word passed on stack before call
; doesn't do anthing but put that item in bc
RRST: org 010h
shld hlsave
pop hl ;hl has return address
pop bc ; BC has data
push hl ; hl back to stack
lhld hlsave ; restor hl
ret
;caller
caller: push bc ;data in bc
rst2
...... This is exactly what I do on both my DMF (8080) and
CUBIX (6809)
OS's.
Understood and like it better too. However it's more natural for
6809 where 8080/z80 is kinda clunky around playing with stack.
I use an 'SSR' macro which looks something like
(DMF/8080):
SSR MACRO
RST 2
DB \1
END
It's fairly easy to fetch this:
SSRENT: STA savea ; Save accumulator
XTHL ; HL = calling address
MOV A,M ; Get service request number
INX H ; Skip for return
XTHL ; Restore HL & new return address
; Now you have the SSR number in A
; most likely you would use it to index into a handler
; table sith something like: (untested)
PUSH H ; Save application HL
MOV L,A ; L = SSR number
MVI H,0 ; Zero high
DAD H ; x2 for two byte entries
PUSH B ; Save BC
LXI B,JMPTAB ; Point to jump table
DAD B ; Offset to table
POP B ; Restore B
MOV A,M ; Get low address
INX H ; Advance to high
MOV H,M ; Get high address
MOV L,A ; Set low address
XTHL ; Restore HL, dest on stack
LDA savea ; Restore A
RET ; Jump to caller
That mkes the code read easier and may be easier to maintain but
that hides whats really there unles you unroll the code.
'JMPTAB' would contain a series of 2-byte addresses of
the individual SSR handlers.
In practice it's usually a bit more complex ... iirc
in my SSR entry I save most of the registers (so that
the handlers don't have to) and switch to my own stack
(but it's been a very long time so I may be mistaken).
In the application code, you can then do things like:
MVI A,'?' ; Get prompt
SSR 3 ; Output to console (two byte OS call)
Dave
Those are interesting ways to deal with it that I'd not have done.
Chalk that up to lack of imagination on my part.
Allison
--
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
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