MMIX Bug Report IS

Table of Content


MMIXware Version


Bug Reported

Initial: 25/1/2012


Martin Ruckert


mmixal can give misleading error messages for the IS instruction.


The following MMIX program
 	LOC	#4000000

Main	JMP	there
there	IS	here

	JMP	further
further IS	near
near	IS	Main
produces the following error messages:
"bug.mms", line 4: the operand is undefined!
"bug.mms", line 7: relative address in location #0000000004000004 is too far away!
(2 errors were found.)
Both error messages are caused by IS instructions. mmixal will create a new symbol with value zero and status undefined in section <Scan the operand field>.

For the first instruction, the correct error message is produced by mixal in section <Do a one-operand operation>.

For the second IS instruction, mmixal will use the undefined operand value in section <Define the label> to set cur_loc, which is then used in section <Fix prior references to this label>. There, the difference between the address of the JMP instruction (#4000004) and the (undefined) value zero of the symbol "near" is considered too big to fit into the 24-bit relative address field of the JMP instruction. Consequently, the second, at least misleading, error message is produced.

Proposed Patch

Since an undefined operand is not allowed for an IS instruction (not even if it is a forward reference), apply the following change to mmixal (section <Define the label>):
if (opcode==IS) { 
if (opcode==IS) { 
  if (val_stack[0].status==undefined) err("the operand is undefined");
After printing the error message, the "err" macro includes a "goto bypass" which will skip the remainder of <Define the label> and the complete section <Do the operation>.

The new error messages are:

"bug.mms", line 4: the operand is undefined!
"bug.mms", line 7: the operand is undefined!
undefined symbol: further
undefined symbol: there


The patch as given above is now part of the sources.

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