Part 4 of the Blue Book--chapter 26 The implementation--in the intro there is this example:
center
^ origin + corner / 2
With these operations
Rectangle center
0 push the value of the receiver's first instance variable (origin) onto the
stack
1 push the value of the receiver's second instance variable (corner) onto the
stack
176 send a binary message with the selector +
119 push the Smalllnteger 2 onto the stack
185 send a binary message with the selector /
124 return the object on top of the stack as the value of the message (center)
In a Workspace, when I doit on 3 + 7/2 I get 5, left, right, unary, binary....rules apply.
What I see here--and please check my reasoning--is a RPN that places things onto the stack via Smalltalk's order of operation rules and then does conventional RPN processing.
In other words, Smalltalk expressions are translated to RPN and then placed on the stack.