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Fixed 36-bit multiplication results in cases of overflow

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Jeff 2017-04-01 15:14:27 -07:00 • committed by Jeff Parsons
commit 1a8c1306d4
4 changed files with 19 additions and 6 deletions
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15
modules/pdp10/lib/cpuops.js
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@ -6531,12 +6531,21 @@ PDP10.split72 = function(res, ext, fExternal)
var sign = ext - (ext % PDP10.INT_LIMIT);
ext = ((ext * 2) % PDP10.WORD_LIMIT) + Math.trunc(res / PDP10.INT_LIMIT);
res = sign + (res % PDP10.INT_LIMIT);
if (fExternal) return res;
var signNew = ext - (ext % PDP10.INT_LIMIT);
if (sign != signNew) {
ext = sign + (ext - signNew);
this.regPS |= PDP10.PSFLAG.AROV;
/*
* I used to restore ext's original sign (ext = sign + (ext - signNew)), but the PDP-10's defined
* behavior for multiplication overflow (ie, whenever both operands are 0o400000000000) is to set both
* res and ext to 0o400000000000.
*
* To quote the original spec for the MUL instruction:
*
* If both operands are -2^35 set Overflow; the double length result stored is -2^70.
*/
res = ext;
if (!fExternal) this.regPS |= PDP10.PSFLAG.AROV;
}
if (fExternal) return res;
this.regEX = res;
return ext;
};