In the VM primitiveSlotAt: / at:Put: mean access to any field (pointer or non pointer, sorry for the confusion Marcus earlier on) of an object. The primitive in the image can be named fieldAt:/fieldAt:put: if it makes more sense for Pharo.
#[0] slotAt: 1 => 0
#(#a) slotAt: 1 => #a
0@2 slotAt: 1 => 0
With Pharo 6 blocks:
| t | t := #tmp. {[ t ] slotAt: 3 . [ t ] slotAt: 4} => { 0 "numArgs". #tmp }��
Right now you can do that with instVarAt: on the contrary to what instVarAt: comment states, and this is wrong.��