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The paper What Every Computer Scientist Should Know About Floating-Point Arithmetic uses the Cray's systems as an example of computers without a guard bit:

Although most modern computers have a guard digit, there are a few (such as Cray® systems) that do not.

Mathematics Written in Sand (William Kahan, 1983) suggests the same (for subtraction, specifically). But the hardware reference manual for the CRAY-1 and CRAY-2 describes how a guard bit is used for the add unit (that implements single-precision arithmetic only). So, I can think of two explanations:

  1. The lack of guard bit affects other Cray systems.
  2. The lack of guard bit affects the software implementation of double-precision arithmetic.

Is one (or both) of these right?

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    What does a "guard bit" do? Is it called something else in some other contexts? Commented 57 mins ago
  • Could you edit your question and quote where the hardware reference manual says that a guard bit is used? E.g. for the Cray-1, it says on page 3-23 that a 49-bit register is used, but the figure suggests that the 49th bit is the leading 1, so I am not entirely sure they use a guard bit. But I may be interpreting it wrong, or there may be a different passage. Commented 14 mins ago

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