a-Imps
An Imp that uses an instruction of the form
mov.i #STEP, *0
where STEP in the a-field is the imp step.
A-imps are sometimes used because they can sometimes bust Gates that increment
the gate's b-field, if the gate isn't carefully constructed. To see this, consider
what happens in the following scenario when an a-imp approaches a Gate: (The asterisk
indicates which instruction has just executed.)
--------------------------------------------------------------------------------
gate: dat xxx, 1234
dat 0,0 ; \\ A-fields must be 0 for all intervening
dat 0,0 ; / instructions
...
jmp 0,>gate ; * increment B-field of gate
--------------------------------------------------------------------------------
mov.i #STEP, *0 ; * imp overwrites the gate
gate: mov.i #STEP, *0
dat 0,0
dat 0,0
...
jmp 0,>gate
--------------------------------------------------------------------------------
mov.i #STEP, *0
gate: mov.i #STEP, *1 ; A-field incremented
dat 0,0
dat 0,0
...
jmp 0,>gate ; * increment gate
--------------------------------------------------------------------------------
mov.i #STEP, *0
gate: mov.i #STEP, *1 ; * imp 'busts the gate' because the A-field of
mov.i #STEP, *1 ; THIS instruction was 0.
dat 0,0
...
jmp 0,>gate
--------------------------------------------------------------------------------
mov.i #STEP, *0
gate: mov.i #STEP, *2 ; incremented
mov.i #STEP, *1
dat 0,0
...
jmp 0,>gate ; * gate executes
--------------------------------------------------------------------------------
mov.i #STEP, *0
gate: mov.i #STEP, *2
mov.i #STEP, *1 ; * malformed imp continues!
mov.i #STEP, *1
dat 0,0
...
jmp 0,>gate
The broken imp instruction mov.i #STEP, *1 will continue until it overwrites the
gating code and has busted the gate. The easiest way to counter this is to make
the A-field after the gate point to somehwere innocuous.
Warriors which are using a-imps are for example: Candy II, RotPendragon 2, Cascade,
Blowrag, Dawn and Digitalis 2002.
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