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(Originated in step5).
It is possible that we encounter an expression like (fx+ 3 4) which can be evaluated at compile time to simply emit

mov eax, 7  ; actually 28 because of fixnum representation

For now, we generate the code

mov eax, 3 ; actually 12 (fixnum)
add eax, 4 ; actually 16 (fixnum)

It is best to keep it the way it is for now (so the compiler code is exercised), but when the compiler code is mature, this can be added as an optimization.

From step2: It would be nice to have a return type for primitives, as also an "acceptable" type. For instance,

(emit-program '(fxadd1 (boolean? 32)))

does not make sense, as fxadd1 should only accept fixnums. If boolean? advertised itself as returning a
boolean, and fxadd1 advertised itself as only accepting a boolean, we could check for this and throw an error at compile time.

From the tutorial exercise 1.4.3:
Consider the expression (if (fx> 3 4) (fxadd1 3) (fxlognot 5)):
fx> first compares it arguments, generates 0 or 1, and then returns #t or #f depending on the result.
if then evaluates the result, compares to #f, and then decides what to do.

The fix would be to have a different definition of emit-expr which would just generate 1 or 0 as the result of the boolean expression. This alternate form would be called only from the test expression of an if, so would not affect results. This would save us in the creation of the boolean.

The standard usage of scheme_entry is that it saves the callee's stack (the stack of the C main() function) in ECX before resetting the stack to the Scheme stack. On return from Scheme (the L_scheme_entry function), ESP is reset from the saved value in ECX.

However, this means that ECX is not available to the code we generate. One possibility would be to:

• Save C stack pointer to ECX (as now)
• Switch stack to the Scheme stack (as now)
• Save ECX to the first element of the Scheme stack.
  • The push instruction will both do this and update ESP.
• ECX is now free to be used by the Scheme-generated code.
  • Note that this means Scheme must restore the stack pointer to exactly where it was upon return. Previously, even if ESP was different, it would be restored from ECX anyway, so it did not matter.

Windows conventions for 32 bit dictate that we can trash EAX, ECX and EDX within a function call; other registers must be saved and restored before returning.

This is the convention used by Visual Studio 2015