supporting mixed 16/32-bit ISA's
Greg McGary
greg@mcgary.org
Tue Jan 22 08:27:00 GMT 2002
Peter.Targett@arccores.com writes:
> I'm particularly interested in CGEN's ability to describe mixed 16/32
> bit ISA's. We have a new ISA at ARC which has a truely intermixed
> 16/32 instruction set - basically, can I describe the ISA in CGEN?
> The 32-bit instructions (and long immediates that can form part of an
> instruction) are actually stored half-word endianized.
I'm not sure what you mean by "half-word endianized". Please clarify.
I did a 16/32 port last year, though unfortunately it's proprietary,
so it's not in the public repo.
The CPU is spec'ed with big-endian bytes, but little-endian bits (lsb
is bit 0). However, I wrote the CGEN port with bit-endian bits (msb
is bit 0). This worked out best since it means the base insn always
occupies bit positions 0..15 (with lsb=0, 16-bit insns would have
the base insn at bits 0..15, but for 32-bit insns, the base insn
would be at bits 16..31)
I can give you some sanitized snippets to show how the port was setup:
;;; Architecture.
(define-arch
(name xxx)
(default-alignment unaligned)
(insn-lsb0? #f)
(machs yyy)
(isas xxx))
;;; Instruction set parameters.
(define-isa
(name xxx)
(default-insn-bitsize 32)
(base-insn-bitsize 32)
(default-insn-word-bitsize 16)
(liw-insns 1)
(parallel-insns 1))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Instruction fields.
(dnf f-op6 "6-bit opcode" () 0 6)
(dnf f-dst "destination register" () 6 5)
(dnf f-src "source register" () 11 5)
(dnf f-uimm16 "16-bit unsigned immediate" () 16 16)
(define-ifield (name f-uimm5)
(comment "5-bit immediate")
(start 11) (length 5) (mode UINT)
(encode (value pc) (if SI (eq SI value -1) (const SI 0) value))
(decode (value pc) (if SI (eq SI value 0) (const SI -1) value))
(minval 0) (maxval 29))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Instruction Operands.
(define-operand (name uimm5)
(comment "5-bit immediate, unsigned with special encoding")
(type h-uimm5)
(index f-uimm5)
(handlers (parse "uimm5")))
(define-operand (name uimm16hi)
(comment "high order 16-bit immediate, unsigned")
(type h-uimm16)
(index f-uimm16)
(handlers (parse "uimm16") (print "uimm16hi")))
(define-operand (name uimm16lo)
(comment "low order 16-bit immediate, unsigned")
(type h-uimm16)
(index f-uimm16)
(handlers (parse "uimm16")))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Instruction definitions.
(define-pmacro (alu-insn mnemonic comment)
(begin
(dni (.sym mnemonic "2i16lo") (.str comment " / 16-bit Immediate Low") ()
(.str mnemonic " $dst,$uimm16lo")
(+ (.sym "OP6_" (.upcase mnemonic) "I") dst (f-uimm5 30) uimm16lo)
(nop) ())
(dni (.sym mnemonic "2i16hi") (.str comment " / 16-bit Immediate High") ()
(.str mnemonic " $dst,$uimm16hi")
(+ (.sym "OP6_" (.upcase mnemonic) "I") dst (f-uimm5 31) uimm16hi)
(nop) ())
(dni (.sym mnemonic "2i5") (.str comment " / 5-bit Immediate") ()
(.str mnemonic " $dst,$uimm5")
(+ (.sym "OP6_" (.upcase mnemonic) "I") dst uimm5)
(nop) ())))
The field f-uimm5 is restricted to the range 0..29 because 30 and 31
are reserved to designate that a 16-bit immediate follows the base
insn. (30 means the immediate is the low 16 bits of a 32-bit word,
and 31 means it is the high 16 bits)
I need special parse handlers for immediates in order to handle %l()
and %h() modifers that pick off the low and high halves of words, and
to range check the restricted 5-bit values. I need special print
handler for uimm16hi in order to shift the value left before printing.
As for why I have these particular values in the define-isa:
(default-insn-bitsize 32)
(base-insn-bitsize 32)
(default-insn-word-bitsize 16)
I don't remember exactly, but that's what worked! 8^)
That should be enough to get you started.
Greg
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