SixtyPical/doc/Analyzing.markdown

Anayzling SixtyPical Programs

-> Tests for functionality "Analyze SixtyPical program"

-> Functionality "Analyze SixtyPical program" is implemented by
-> shell command "bin/sixtypical analyze %(test-file)"

Analysis determines what storage locations have been modified by a routine.

| reserve byte score
| routine main {
|   lda #4
|   sta score
| }
= main ([])
=   A: UpdatedWith (Immediate 4)
=   NamedLocation Nothing "score": UpdatedWith A

A routine cannot expect registers which a called routine does not preserve, to be preserved.

| assign byte border_colour 4000
| reserve byte score
| routine update_score
| {
|   lda #8
|   sta score
| }
| routine main {
|   lda #4
|   jsr update_score
|   sta border_colour
| }
? routine 'main' does not preserve 'A'

But if it does it can.

| assign byte border_colour 4000
| reserve byte score
| routine update_score
| {
|   ldx score
|   inx
|   stx score
| }
| routine main {
|   lda #4
|   jsr update_score
|   sta border_colour
| }
= main ([])
=   A: UpdatedWith (Immediate 4)
=   X: PoisonedWith (Immediate 1)
=   NamedLocation Nothing "border_colour": UpdatedWith A
=   NamedLocation Nothing "score": PoisonedWith X
= 
= update_score ([])
=   X: UpdatedWith (Immediate 1)
=   NamedLocation Nothing "score": UpdatedWith X

We can't expect to stay named variables to stay unmodified either.

| reserve byte score
| routine update_score
| {
|   lda #8
|   sta score
| }
| routine main {
|   jsr update_score
|   lda score
| }
? routine 'main' does not preserve 'NamedLocation Nothing "score"'

What the solution to the above is to notate update_score as intentionally modifying score, as an "output" of the routine.

| assign byte border_colour 4000
| reserve byte score
| routine update_score outputs (score)
| {
|   lda #8
|   sta score
| }
| routine main {
|   ldx score
|   jsr update_score
|   ldx score
| }
= main ([])
=   A: PoisonedWith (Immediate 8)
=   X: UpdatedWith (NamedLocation Nothing "score")
=   NamedLocation Nothing "score": UpdatedWith A
= 
= update_score ([NamedLocation Nothing "score"])
=   A: UpdatedWith (Immediate 8)
=   NamedLocation Nothing "score": UpdatedWith A

Routines can name registers as outputs.

| reserve byte score
| routine update_score
| {
|   lda #8
| }
| routine main {
|   jsr update_score
|   sta score
| }
? routine 'main' does not preserve 'A'

| reserve byte score
| routine update_score outputs (.a)
| {
|   lda #8
| }
| routine main {
|   jsr update_score
|   sta score
| }
= main ([])
=   A: UpdatedWith (Immediate 8)
=   NamedLocation Nothing "score": UpdatedWith A
= 
= update_score ([A])
=   A: UpdatedWith (Immediate 8)

If a location is poisoned in either branch of an if, it is poisoned after the if.

| reserve byte score
| routine update_score
| {
|   if beq {
|      lda #8
|   } else {
|      ldx #8
|   }
| }
| routine main {
|   lda #4
|   jsr update_score
|   sta score
| }
? routine 'main' does not preserve 'A'

| reserve byte score
| routine update_score
| {
|   if beq {
|      ldx #8
|   } else {
|      lda #8
|   }
| }
| routine main {
|   lda #4
|   jsr update_score
|   sta score
| }
? routine 'main' does not preserve 'A'

| reserve byte score
| routine update_score
| {
|   lda #4
|   sta score
| }
| routine main {
|   lda #4
|   if beq {
|      jsr update_score
|   } else {
|      ldx #3
|   }
|   sta score
| }
? routine 'main' does not preserve 'A'

| reserve byte score
| routine update_score
| {
|   lda #4
|   sta score
| }
| routine main {
|   lda #4
|   if beq {
|      ldx #3
|   } else {
|      jsr update_score
|   }
|   sta score
| }
? routine 'main' does not preserve 'A'

Poisoning a high byte or low byte of a word poisons the whole word.

| reserve word score
| reserve byte temp
| routine update_score
| {
|   ldx #4
|   stx <score
| }
| routine main {
|   jsr update_score
|   lda >score
|   sta temp
| }
? routine 'main' does not preserve 'NamedLocation Nothing "score"'