Add sketches/notes re: subroutines and a call stack

notes/2023-08-12--dev-notes.md

@@ -0,0 +1,60 @@
+# Dev notes — 2023-08-12
+
+Brainstorming/sketching around subroutines with a return stack...
+
+    ; need an instruction for IP → A i guess?
+    ; ideally…
+    ; but a jump table would work
+    ; put that at beginning of the code
+    ; then store numbers for subroutine labels in a designated memory slot
+
+    lda $1
+    sto $19 ; contains ID # for the next fn to jump to
+
+    @jump_table
+    hop $1
+      jmp @jt2
+    jmp @example_computation
+    @jt2
+    hop $2
+      ; jmp @jt3
+      nop
+    ; etc …
+    jmp @end
+
+    @example_computation
+    lda 5
+    sto $20
+    lda 3
+    sto $21
+    ; $19 still has the # for this routine
+    ; but let’s pretend it doesn’t and demonstrate updating it
+    lda $1
+    sto $19
+    jmp @greater?
+
+    ; call with numbers to test in $20 and $21
+    ; result is stored in acc
+    @greater?
+    ; lda ($20)
+    ; sub ($21)
+    ; todo…
+    ; wouldn’t it be great to have a “hop if neg” op…
+    ; do we have to just subtract numbers until we get 0?
+
+    ; no!
+    ; here’s an approach that’s at least better than that
+    lda ($21)
+    sto $22 ; stash
+    @loop
+    lda ($21)
+    sub $1
+    sto $22 ; stash
+    sub ($20)
+    hop $0
+      jmp @loop
+    sto $1
+    jmp $jmp_table
+    ; ok this isn’t quite it… we also need to chexk if we hit 0 by just deceementinf and if so retuen 0
+
+    jmp @jump_table

test-programs/subroutines.asm

@@ -0,0 +1,62 @@
+;; sketching around subroutines with return stack
+
+; jump table
+; ----------
+; each function has a label
+; and you can get to it with `JMP @label`
+; ...but you can't push a label onto a stack
+; so instead we give each function a number
+; and stash that number in $24 when jumping to a subroutine
+; then when the subroutine ends, it jumps here
+; where that number gets mapped back onto the function's label
+
+lda $1
+sto $19 ; contains ID # for the next fn to jump to
+
+@jump_table
+hop $1
+  jmp @jt2
+jmp @example_computation
+@jt2
+hop $2
+   ; jmp @jt3
+   nop
+; etc …
+jmp @end
+
+@example_computation
+lda 5
+sto $20
+lda 3
+sto $21
+; $19 still has the # for this routine
+; but let’s pretend it doesn’t and demonstrate updating it
+lda $1
+sto $19
+jmp @greater?
+
+; call with numbers to test in $20 and $21
+; result is stored in acc
+@greater?
+; lda ($20)
+; sub ($21)
+; todo…
+; wouldn’t it be great to have a “hop if neg” op…
+; do we have to just subtract numbers until we get 0?
+
+; no!
+; here’s an approach that’s at least better than that
+lda ($21)
+sto $22 ; stash
+@loop
+lda ($21)
+sub $1
+sto $22 ; stash
+sub ($20)
+hop $0
+  jmp @loop
+sto $1
+jmp $jmp_table
+; ok this isn’t quite it… we also need to chexk if we hit 0 by just deceementinf and if so retuen 0
+
+jmp @jump_table