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22 lines
1.0 KiB
Markdown
22 lines
1.0 KiB
Markdown
## Known differences between SimH, 11/70, and w11a
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### SimH: State of N and Z and registers after a `DIV` abort with `V=1`
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The state of the N and Z condition codes is specified as unspecified for
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the `DIV` instruction when V=1 is set after a zero divide or an overflow
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condition.
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See [1979 processor handbook](http://www.bitsavers.org/pdf/dec/pdp11/handbooks/PDP11_Handbook1979.pdf) on page 75.
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The 11/70 leaves the N and Z condition codes and the result registers in a
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state depending on the abort point in the microcode state flow. That results
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in sometimes surprising settings.
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SimH returns Z=0 and N based on the sign of the full 32-bit result, as can be
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easily determined by xor'ing of the sign bits of dividend and divisor.
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xxdp program `ekbbf0` test 014 checks the exact 11/70 behavior to verify the
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divide logic and is modified (see [patch](../tools/xxdp/ekbbf0_patch_1170.scmd)).
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w11 also returns Z=0 and N based on the sign of the full 32-bit result, this
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is documented as [w11 known difference](w11a_diff_70_div_after_v1.md).
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