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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Errata for: Warren's Abstract Machine: A Tutorial Reconstruction Hassan Ait-Kaci MIT Press, Cambridge, MA 1991 ISBN 0-262-51058-8 (paper) ISBN 0-262-01123-9 (cloth) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% I am enclosing below the most up-to-date list of typos, bugs, and their - easy - fixes... Anything else that you will find, please report back to me. Who knows, one day when my stack is finally empty (ha!), I'll work on a second edition with extensions. In the mean time, please accept my apologies for your painful reading. On the other hand, my book's bugs and typos are a wonderful indicator of who did or not actually try to implement the code therein! These bug reports and fixes are to be credited to James Anhalt III (anhalt@cs.ucla.edu), Dan Friedman (dfried@cs.indiana.edu), Michael Levy (mlevy@csr.uvic.ca), Donald A. Smith (dsmith@chaos.cs.brandeis.edu), and Neng Fa Zhou (zhou@csce.kyushu-u.ac.jp). Big thanks to all. -hak ___________________________________________________________________ Hassan Ait-Kaci, Professor ___________________________________________________________________ School of Computing Science phone: +1 (604) 291 55 89 Simon Fraser University fax: +1 (604) 291 30 45 Burnaby, British Columbia email: hak@cs.sfu.ca V5A 1S6, Canada url: http://www.isg.sfu.ca/~hak/ ___________________________________________________________________ %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% In the definition of get_structure (fig 2.6, page 13) S should be initialized to 1 before exiting get_structure in either READ or WRITE modes. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% There is a problem with stack frames allocation. Namely, the book defines ALLOCATE as: (*) if E > B then newE <- E + CODE[STACK[E + 1] - 1] + 2 else ... While this should be: (**) if E > B then newE <- E + CODE[CP - 1] + 2 else ... The following code shows why: a/0 allocate ... call b/0,1 L1 ... b/0 allocate ... call c/0,3 L2 ... c/0 allocate ... Now when b/0 calls c/0 the stack should look like: |-------| |CE: |0 <- a's environment |-------| |CP: |1 |-------| |Y1: |2 |-------| E -> |CE: 0 |3 <- b's environment |-------| |CP: L1 |4 |-------| |Y1: |5 |-------| |Y2: |6 |-------| |Y3: |7 |-------| | |8 <- we want c's environment to start here |-------| | |9 |-------| CP = L2 P = c/0 So when c/0 executes allocate ... With (*) the new value of E would be: E = 3 + CODE[STACK[3 + 1] - 1] + 2 E = 3 + CODE[L1 - 1] + 2 E = 3 + 1 + 2 E = 6 which overwrites Y2 and Y3 since it uses the 1 from a/0 Whereas (**) gives us: E = 3 + CODE[CP - 1] + 2 E = 3 + CODE[L2 - 1] + 2 E = 3 + 3 + 2 E = 8 which is right since b/0 wants to save 3 locals This same problem exists in all the instructions that deal with allocating new stack frames. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% As described UNIFY_CONSTANT does not increment the S register, this makes it very hard to read structures with constants which are not the last argument. Easy fix... %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% When allocating a new choice or environment frame on the stack, one should use CP (the continuation pointer) insteads of E+1 (the stored continuation pointer) to find out the number of Y variables to preserve in the previous environment frame. This is because the continuation pointer is stored on the stack only if an ALLOCATE instruction is used and so only CP has the real value. Thus, instead of if (E > B) NewB = E + *(((int *) *(E+1))-1) + 2; else NewB = B + *B + FIXED_CHOICE_FRAME_SIZE; one should use code like if (E > B) NewB = E + *(CP-1) + 2; else NewB = B + *B + FIXED_CHOICE_FRAME_SIZE; %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% The try, retry, and trust instructions reset the HB register to an incorrect value. The problem is that n is computed from the original value of B (n <- STACK[B]). Hence n is no longer valid when HB is re-loaded. The correct code is: HB <- STACK[B+STACK[B]+6] There is a more subtle related bug that usually doesn't matter very much: both cut and neck_cut should also reset HB. If they do not, some uneccessary trailing will occur. This normally doesnt matter too much (aside from a small performance penalty), but it does turn out to be a problem if you try to implement Older and Rummel's incremental garbage collection algorithm, because you end up with dangling trail references to collected heap storage. O&R's algorithm relies on knowing that there are no trail references below a certain point into the tip of the heap. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%