From: bugzilla-daemon@libre-riscv.org
To: libre-riscv-dev@lists.libre-riscv.org
Subject: [libre-riscv-dev] [Bug 267] The efficiency of adder/subtractor
Date: Thu, 26 Mar 2020 23:10:07 +0000
Message-ID: <bug-267-13-VjjJk21Mdy@http.bugs.libre-riscv.org/> (raw)
In-Reply-To: <bug-267-13@http.bugs.libre-riscv.org/>
http://bugs.libre-riscv.org/show_bug.cgi?id=267
--- Comment #1 from Luke Kenneth Casson Leighton <lkcl@lkcl.net> ---
(In reply to Jock Tanner from comment #0)
> Actually I was thinking about what Harris&Harris suggested in their famous
> book. You just have to have an inverter on one operand input of an adder.
> The two's complement of that operand can be achieved via the carry input.
yes, interestingly, michael and i are reviewing in the simulator, various
implementations (microwatt, qemu, pear_pc), they all seem to do this
http://bugs.libre-riscv.org/show_bug.cgi?id=186#c130
it looks like power was extremely well designed, and microwatt as well,
to basically "translate" opcodes into "everything is an ADD, but you
optionally do add1, add0, add-carry, occasionally invert op1,
oh and farm out carry to different places, or set carry to 0, or 1" etc. etc.
it's quite neat.
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Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-26 22:05 [libre-riscv-dev] [Bug 267] New: " bugzilla-daemon
2020-03-26 23:10 ` bugzilla-daemon [this message]
2020-04-07 5:23 ` [libre-riscv-dev] [Bug 267] " bugzilla-daemon
2020-04-07 9:48 ` bugzilla-daemon
2020-04-07 11:39 ` bugzilla-daemon
2020-04-07 11:48 ` bugzilla-daemon
2020-04-07 13:15 ` bugzilla-daemon
2020-04-07 13:36 ` bugzilla-daemon
2020-04-07 13:38 ` bugzilla-daemon
2020-04-07 14:44 ` bugzilla-daemon
2020-04-07 15:14 ` whygee
2020-04-07 15:25 ` Jacob Lifshay
2020-04-07 15:46 ` whygee
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