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Added some comments regarding 72-bit division.

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Jeff 2017-04-02 11:04:20 -07:00 • committed by Jeff Parsons
commit defa6cd038
1 changed files with 15 additions and 6 deletions
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21
modules/pdp10/lib/cpuops.js
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@ -6241,6 +6241,10 @@ PDP10.doDIV = function(src, dst, ext)
{
var fNegQ = false, fNegR = false;
/*
* Perform all the same up-front checks that the PDP-10 performs; if we do our job correctly, then it
* will be impossible for the actual division operation to overflow (the asserts below should never fire).
*/
if (ext === undefined) {
if (!src) {
this.regPS |= PDP10.PSFLAG.DCK | PDP10.PSFLAG.AROV;
@ -6257,14 +6261,19 @@ PDP10.doDIV = function(src, dst, ext)
}
}
/*
* We're done with the PDP-10's "dual sign" operands now; produce a (unified) signed 72-bit dividend.
*/
dst = PDP10.merge72.call(this, dst, ext);
ext = this.regEX;
/*
* Make all the inputs positive now, to keep the division simple. Fix up the results when we're done.
*/
if (src > PDP10.INT_MASK) {
src = PDP10.WORD_LIMIT - src;
fNegQ = !fNegQ;
}
if (ext > PDP10.INT_MASK) {
if (dst) {
ext = PDP10.WORD_MASK - ext;
@ -6335,10 +6344,10 @@ PDP10.doDIV = function(src, dst, ext)
* number (base 2^18). Each individual multiplication of these 18-bit "digits" will produce
* a result within 2^36, well within JavaScript integer accuracy.
*
* PDP-10 "DAKAK" Diagnostic Notes
* -------------------------------
* PDP-10 Diagnostic Notes
* -----------------------
*
* The DAKAK diagnostic contains the following code:
* The "DAKAK" diagnostic contains the following code:
*
* 036174: 200240 043643 MOVE 5,43643 ; [43643] = 400000000000
* 036175: 200300 043603 MOVE 6,43603 ; [43603] = 777777777777
@ -6425,7 +6434,7 @@ PDP10.doMUL = function(dst, src, fTruncate, fExternal)
/*
* Special code for IMUL. I originally tried to avoid calling split72() in this case, but the PDP-10
* still appears to be splitting the result of the multiplication into separately signed 36-bit values,
* so the simplest solution is to call split72() in both cases.
* so the simplest solution is to call split72() in all cases.
*/
res = this.regEX;
if (!fExternal) {
@ -6435,7 +6444,7 @@ PDP10.doMUL = function(dst, src, fTruncate, fExternal)
* Set Overflow if the product is >= 2^35 or < -2^35 (ie, if the high order word of the double
* length product is not null); the high order word is lost.
*
* However, when the DAKAL diagnostic performs a sequence like this:
* However, when the "DAKAL" diagnostic performs a sequence like this:
*
* 00=000000000000 01=000000000000 02=000000000000 03=400000036755
* 04=400000000000 05=000000000003 06=400000000000 07=000000000004