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[bug-mdk] [bug #5652] Assembler shouldn't mask index field
From: |
nobody |
Subject: |
[bug-mdk] [bug #5652] Assembler shouldn't mask index field |
Date: |
Tue, 30 Sep 2003 22:41:23 -0400 |
User-agent: |
Nutscrape/1.0 (CP/M; 8-bit) |
=================== BUG #5652: FULL BUG SNAPSHOT ===================
http://savannah.gnu.org/bugs/?func=detailbug&bug_id=5652&group_id=118
Submitted by: joelh Project: GNU MIX Development Kit
Submitted on: Wed 10/01/2003 at 02:41
Category: None Severity: 5 - Major
Bug Group: None Resolution: None
Assigned to: None Status: Open
Summary: Assembler shouldn't mask index field
Original Submission: The assembler, in mix_ins_to_word (and
mix_ins_to_word_uncheck) presently truncates the index field to three bits.
As far as I can tell, this serves no useful purpose. It doesn't restrict the
index to valid index registers, since 7 is still passed. It doesn't restrict
input, since index register values > 7 are modified to other values (which are
pretty much useless), rather than generating an error. It isn't required to
make a valid word, since an entire byte is available in the I field.
There's an old tradition on machines similar to the MIX (such as the PDP-4, IBM
360, etc) of holding data in unused portions of instructions, or (if you can
wrangle it) used portions of instructions. Or, if you don't have anything
useful to store, then your initials. ANDing the index field with 7 prevents
this, and is unnecessary.
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For detailed info, follow this link:
http://savannah.gnu.org/bugs/?func=detailbug&bug_id=5652&group_id=118
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