Here is the microcode language, it emulates datapath level operations. It's a small language that supports a limited set of commands that mutate the state of a virtual BETA cpu with ram.
(add-instruction '(ADD RA RB RC) 'OP #b100000
"The contents of register Ra are added to the
contents of register Rb and the 32-bit sum is
written to Rc. This instruction computes no carry or
overflow information. If desired then this can be
computed through explicit compare instructions."
'((inc-pc)
(set-reg rc (+ (reg ra) (reg rb)))))Here are some of the microcode commands.
inc-pc increments the program counter,
(reg ra) selects register ra from the register file
(set-var) can be used for naming variables
(set-mem) is used to set RAM locations
The languages can also handle expressions:
(+ a b) and other arithmetic operations emulate the ALU
The machinery is an evalulator like the ones found in SICP