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+# Powdr Assembly (Powdr-Asm)
+
+Powdr-Asm is used to define instruction sets and write programs that depend on
+control flow, with the aim of generating ZKPs about them.
+Powdr-Asm follows the execution trace table model shared by many proof systems.
+In fact, Powdr-Asm extends Polygon Hermez' [PIL and
+zkASM](https://zkevm.polygon.technology/docs/category/polynomial-identity-language).
+In such a model, the execution is represented by a two-dimensional fixed-size
+table, where columns represent the state (for example, registers or
+memory) and each row represents a step in time for a given execution.
+Constraints specify how the state evolves from any step to the next.
+
+As an example, let's apply that model to a program that computes
+the factorial of a number. Our state consists of two registers:
+
+- CNT - a counter
+- ACC - the answer accumulator
+
+The first row represents the initial state of our program, that is, `PC =
+0`, `CNT = <input>`, `ACC = 1`.
+From then on, we compute the subsequent rows with the following pseudocode:
+
+START:
+  Is CNT = 0?
+  If `yes` go to `END`
+  ACC := ACC * CNT
+  CNT := CNT - 1
+  Go to START
+END:
+  ACC equals `factorial(input)`
+
+We can compute the following execution trace with 5 as input:
+
+| CNT | ACC |
+|-----|-----|
+| 5   | 1   |
+| 4   | 5   |
+| 3   | 20  |
+| 2   | 60  |
+| 1   | 120 |
+| 0   | **120** |
+
+Note that the height of the table limits the length of the trace.
+
+## ISA
+
+There are three constructs that can be used to define an instruction set:
+
+- Registers
+- Macros
+- Instructions
+
+### Registers
+
+Registers are basic state components that can store a single number.
+There are three types of registers:
+
+- Program Counter (PC). Used for control flow.
+- Assignment registers. Used by parametric instructions and for assignments.
+- General registers. Used by program logic.
+
+The registers are declared in the following way:
+
+```asm
+reg pc[@pc];
+reg X[<=];
+reg A;
+```
+
+In the snippet above, the `@pc` annotation means that `pc` is implicitly
+incremented by one for instructions that do not explicitly assign it.  The `<=` annotation
+means `X` is an assignment register and can be used by parametric instructions
+and for assignments.
+Finally, `A` is a general purpose register and can be used by program logic.
+
+### Instructions
+
+Instructions are predicates that define constraints that are enforced only when the
+instruction is invoked. The constraints may refer to registers on the execution
+trace line that it is invoked and/or the next line.
+If the next line version of a register is used, for example `X'`, it must be in
+the LHS of the expression, where the RHS contains no next registers.
+If an instruction has formal parameters, they must be either literals or
+assignment registers.
+
+```asm
+instr ADD X, Y -> Z { X + Y = Z }
+```
+
+The above instruction can be read as "If `Z` is the same as `X + Y`, then `ADD
+X Y` returns `Z`".
+As an analogy, this is similar to Prolog predicates.
+
+
+### Macros
+
+Macros can be used to build expressions which are inlined at the call site.
+
+```asm
+macro if_then_else(condition, true_value, false_value) {
+    condition * true_value + (1 - condition) * false_value
+};
+```
+
+The macro above can be used to simplify if-then-else arithmetic expressions.
+
+### Factorial, again
+
+We now have enough constructs to create a simple ISA and implement our
+Factorial program.
+
+```asm
+pil {
+    col witness XInv;
+    col witness XIsZero;
+    XIsZero * (1 - XIsZero) = 0;
+    XIsZero = 1 - X * XInv;
+    XIsZero * X = 0;
+}
+
+macro if_then_else(condition, true_value, false_value) {
+    condition * true_value + (1 - condition) * false_value
+};
+
+macro jump_to(target) { pc' = target; };
+
+macro jump_to_if(condition, target) {
+    jump_to(if_then_else(condition, target, pc + 1));
+};
+
+reg pc[@pc];
+reg X[<=];
+reg CNT;
+reg ACC;
+
+instr jmpz X, l: label { jump_to_if(XIsZero, l) }
+instr jmp l: label { jump_to(l) }
+instr dec_CNT { CNT' = CNT - 1 }
+instr assert_zero X { XIsZero = 1 }
+
+CNT <=X= ${ ("input", 0) };
+
+start::
+ jmpz CNT, end;
+ ACC <=X= ACC + ${ ("input", CNT) };
+ dec_CNT;
+ jmp start;
+
+end::
+```
+
+The beginning of the snippet above is a necessary gadget that computes whether
+`X` is 0 at a certain point.
+
+The macros, registers and instructions define our ISA, and can be used to write
+any programs that only need these features.
+
+Finally we have the program itself, which can be either manually written or
+automatically compiled from some higher level language.