Fix calling issues

7 files changed, 310 insertions, 556 deletions

diff --git a/asm/ast_compiler_bootstrap.atk16 b/asm/ast_compiler_bootstrap.atk16
index 7036291..18a345a 100644
--- a/asm/ast_compiler_bootstrap.atk16
+++ b/asm/ast_compiler_bootstrap.atk16
@@ -2,7 +2,9 @@
 
 @use ext_std:*
 
-@let sp              RH
+@let sp              RH   ; stack pointer (points to empty slot at top of stack)
+@let fp              RG   ; frame pointer (points to base of currently active frame)
+
 @let vector_table    0x10
 @let vt_ISR0         0x10 ; ISR0
 @let vt_ISR1         0x11 ; ISR1
@@ -30,6 +32,7 @@
 @address 0x0
   ldi vt_stack_addr RA
   ldr RA SP
+  mov SP FP
   jpi program_segment
 
 @address vector_table
diff --git a/asm/test_py_src.atk16 b/asm/test_py_src.atk16
index 2d8e311..b605dad 100644
--- a/asm/test_py_src.atk16
+++ b/asm/test_py_src.atk16
@@ -2,7 +2,9 @@
 
 @use ext_std:*
 
-@let sp              RH
+@let sp              RH   ; stack pointer (points to empty slot at top of stack)
+@let fp              RG   ; frame pointer (points to base of currently active frame)
+
 @let vector_table    0x10
 @let vt_ISR0         0x10 ; ISR0
 @let vt_ISR1         0x11 ; ISR1
@@ -30,6 +32,7 @@
 @address 0x0
   ldi vt_stack_addr RA
   ldr RA SP
+  mov SP FP
   jpi program_segment
 
 @address vector_table
@@ -77,47 +80,39 @@
   23
 @label text_mem_addr_p
   25
-@label int_1
+@label int_0
   65
-@label int_5
+@label int_6
   26
 
-; <ast.FunctionDef object at 0x1048ce500>
+; <ast.FunctionDef object at 0x104f1f370>
 @label nth_letter
 ; stack frame with offsets
-; return_address_0 3
-; n 4
-; <ast.Return object at 0x1048ce440>
-; <ast.Add object at 0x1045f6ad0>
-; BinOp lhs <ast.BinOp object at 0x1048ce410>
-; <ast.Name object at 0x1048ce3e0> (n)
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 4 RB
-  add RA RB RA
-  ldr RA RA
-  spu RA
-; BinOp rhs <ast.BinOp object at 0x1048ce410>
-; <ast.Constant object at 0x1048ce3b0>
-  ldi int_1 RA
-  ldr RA RA
-  spu RA
-  spo RB
-  spo RA
-  add RA RB RA
-  spu RA
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 3 RB
-  add RA RB RA
-; stack address of return address is stored in reg RA
-  spo RB
-; return value is stored in reg RB
-  mov RA SP
-  ldr RA RA
-  spu RB
+; n 0
+; <ast.Return object at 0x104f1f430>
+; <ast.Add object at 0x104ed2c20>
+; BinOp lhs <ast.BinOp object at 0x1051b52a0>
+; <ast.Call object at 0x1051b5270>
+; Builtin call ord [<ast.Constant object at 0x1051b51e0>]
+; <ast.Constant object at 0x1051b51e0>
+  ldi int_0 RC
+  ldr RC RC ; int_0 = 65
+  spu RC
+; BinOp rhs <ast.BinOp object at 0x1051b52a0>
+; <ast.Name object at 0x1051b51b0> (n)
+  ldi 0 RC
+  add FP RC RC
+  ldr RC RC
+  spu RC
+  spo RD
+  spo RC
+  add RC RD RC
+  spu RC
+; return value is on top of stack
 ; return from function
-  jpr RA
+  subi FP 1 FP
+  ldr FP RB
+  jpr RB
 
 @label main
 ; stack frame with offsets
@@ -125,186 +120,182 @@
 ; text_mem_addr 1
 ; i 2
   addi SP 3 SP
-; <ast.Assign object at 0x1048ce860>
-; assigning graphics_mode_addr at stack segment + 0
+; <ast.Assign object at 0x104f1ffd0>
+; assigning graphics_mode_addr at FP + 0
 ; evaluating value to be assigned
-; <ast.Call object at 0x1048ce800>
-; Builtin call load [<ast.Name object at 0x1048ce770>]
-; <ast.Name object at 0x1048ce770> (GRAPHICS_MODE_ADDR_P)
+; <ast.Call object at 0x104f1fa00>
+; Builtin call load [<ast.Name object at 0x104f1f970>]
+; <ast.Name object at 0x104f1f970> (GRAPHICS_MODE_ADDR_P)
   ldi graphics_mode_addr_p RC
   ldr RC RC
   spu RC
+; (load) dereferencing top of stack
   spo RC
   ldr RC RC
   spu RC
-; assigning value to stack segment + 0
-  ldi vt_stack_addr RA
-  ldr RA RA
+; assigning stack address (FP + 0) := top of stack
+  spo RA
   ldi 0 RB
-  add RA RB RA
-  spo RB
-  str RB RA
-; <ast.Assign object at 0x1048ce740>
-; assigning text_mem_addr at stack segment + 1
+  add RB FP RB
+  str RA RB
+; <ast.Assign object at 0x104f1f940>
+; assigning text_mem_addr at FP + 1
 ; evaluating value to be assigned
-; <ast.Call object at 0x1048ce6e0>
-; Builtin call load [<ast.Name object at 0x1048ce650>]
-; <ast.Name object at 0x1048ce650> (TEXT_MEM_ADDR_P)
+; <ast.Call object at 0x104f1f610>
+; Builtin call load [<ast.Name object at 0x104f1f550>]
+; <ast.Name object at 0x104f1f550> (TEXT_MEM_ADDR_P)
   ldi text_mem_addr_p RC
   ldr RC RC
   spu RC
+; (load) dereferencing top of stack
   spo RC
   ldr RC RC
   spu RC
-; assigning value to stack segment + 1
-  ldi vt_stack_addr RA
-  ldr RA RA
+; assigning stack address (FP + 1) := top of stack
+  spo RA
   ldi 1 RB
-  add RA RB RA
-  spo RB
-  str RB RA
-; <ast.Call object at 0x1048ce5f0>
-; Builtin call store [<ast.Name object at 0x1048ce560>, <ast.Name object at 0x1048ce530>]
-; <ast.Name object at 0x1048ce560> (graphics_mode_addr)
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 0 RB
-  add RA RB RA
+  add RB FP RB
+  str RA RB
+; <ast.Call object at 0x104f1f4f0>
+; Builtin call store [<ast.Name object at 0x104f1f3d0>, <ast.Name object at 0x104f1f3a0>]
+; <ast.Name object at 0x104f1f3d0> (graphics_mode_addr)
+  ldi 0 RA
+  add FP RA RA
   ldr RA RA
   spu RA
-; <ast.Name object at 0x1048ce530> (TEXT_MODE)
+; <ast.Name object at 0x104f1f3a0> (TEXT_MODE)
   ldi text_mode RA
   ldr RA RA
   spu RA
   spo RB
   spo RA
   str RB RA
-; <ast.Assign object at 0x1048ce350>
-; assigning i at stack segment + 2
+  ldi 0 RA
+  spu RA
+; popping free-standing Expr result from stack
+  spo RA
+; <ast.Assign object at 0x1051b5150>
+; assigning i at FP + 2
 ; evaluating value to be assigned
-; <ast.Constant object at 0x1048ce2f0>
+; <ast.Constant object at 0x1051b50f0>
   ldi 0 RC
   spu RC
-; assigning value to stack segment + 2
-  ldi vt_stack_addr RA
-  ldr RA RA
+; assigning stack address (FP + 2) := top of stack
+  spo RA
   ldi 2 RB
-  add RA RB RA
-  spo RB
-  str RB RA
-; <ast.Call object at 0x1048ce290>
-; Builtin call asm [<ast.Constant object at 0x1048ce200>]
-  ldi 0b111111111 RG
-; <ast.While object at 0x1048ce1d0>
-@label while_test_2
-; <ast.Name object at 0x1048ce170> (i)
-  ldi vt_stack_addr RB
-  ldr RB RB
-  ldi 2 RC
-  add RB RC RB
+  add RB FP RB
+  str RA RB
+; <ast.While object at 0x1051b50c0>
+@label while_test_1
+; <ast.Compare object at 0x1051b5090>
+; <ast.Name object at 0x1051b5060> (i)
+  ldi 2 RB
+  add FP RB RB
   ldr RB RB
   spu RB
-; <ast.Constant object at 0x1048ce140>
-  ldi int_5 RB
-  ldr RB RB
+; <ast.Constant object at 0x1051b5030>
+  ldi int_6 RB
+  ldr RB RB ; int_6 = 26
   spu RB
+; Comparing <ast.Name object at 0x1051b5060> <ast.Lt object at 0x104ed2470> <ast.Constant object at 0x1051b5030>
   spo RC
   spo RB
   sub RB RC RB
-  bri carry Compare_true_6
+  bri carry Compare_true_4
   ldi 0 RB
   spu RB
-  jpi Compare_end_7
-@label compare_true_6
+  jpi Compare_end_5
+@label compare_true_4
   ldi 1 RB
   spu RB
-@label compare_end_7
+@label compare_end_5
   spo RA
   addi RA 0 RA
-  bri zero While_else_3
-; <ast.Call object at 0x1048ce0e0>
-; Builtin call store [<ast.BinOp object at 0x1048ce050>, <ast.Call object at 0x1048cdd80>]
-; <ast.Add object at 0x1045f6ad0>
-; BinOp lhs <ast.BinOp object at 0x1048ce050>
-; <ast.Name object at 0x1048ce020> (text_mem_addr)
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 1 RB
-  add RA RB RA
+  bri zero While_else_2
+; <ast.Call object at 0x1051b4fd0>
+; Builtin call store [<ast.BinOp object at 0x1051b4f40>, <ast.Call object at 0x1051b4eb0>]
+; <ast.Add object at 0x104ed2c20>
+; BinOp lhs <ast.BinOp object at 0x1051b4f40>
+; <ast.Name object at 0x1051b4f10> (text_mem_addr)
+  ldi 1 RA
+  add FP RA RA
   ldr RA RA
   spu RA
-; BinOp rhs <ast.BinOp object at 0x1048ce050>
-; <ast.Name object at 0x1048cdff0> (i)
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 2 RB
-  add RA RB RA
+; BinOp rhs <ast.BinOp object at 0x1051b4f40>
+; <ast.Name object at 0x1051b4ee0> (i)
+  ldi 2 RA
+  add FP RA RA
   ldr RA RA
   spu RA
   spo RB
   spo RA
   add RA RB RA
   spu RA
-; <ast.Call object at 0x1048cdd80>
+; <ast.Call object at 0x1051b4eb0>
 ; Call function nth_letter
+  spu FP
   ldi 0 RA
   spu RA
-; <ast.Name object at 0x1048cdd20> (i)
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 2 RB
-  add RA RB RA
+; <ast.Name object at 0x1051b4e50> (i)
+  ldi 2 RA
+  add FP RA RA
   ldr RA RA
   spu RA
+  subi SP 2 FP
 ; set up return address and jump to subroutine
-  subi SP 2 RA
-  lpc RB
-  addi RB 3 RB
-  str RB RA
+  lpc RA
+  addi RA 4 RA
+  str RA FP
+  addi FP 1 FP
   jpi nth_letter
+  spo RA
+  mov FP SP
+  spo FP
+  spu RA
   spo RB
   spo RA
   str RB RA
-; <ast.Assign object at 0x1048ccc70>
-; assigning i at stack segment + 2
+  ldi 0 RA
+  spu RA
+; popping free-standing Expr result from stack
+  spo RA
+; <ast.Assign object at 0x1051b4e20>
+; assigning i at FP + 2
 ; evaluating value to be assigned
-; <ast.Add object at 0x1045f6ad0>
-; BinOp lhs <ast.BinOp object at 0x1048cda50>
-; <ast.Name object at 0x1048cd750> (i)
-  ldi vt_stack_addr RC
-  ldr RC RC
-  ldi 2 RD
-  add RC RD RC
+; <ast.Add object at 0x104ed2c20>
+; BinOp lhs <ast.BinOp object at 0x1051b4dc0>
+; <ast.Name object at 0x1051b4d90> (i)
+  ldi 2 RC
+  add FP RC RC
   ldr RC RC
   spu RC
-; BinOp rhs <ast.BinOp object at 0x1048cda50>
-; <ast.Constant object at 0x1048cd720>
+; BinOp rhs <ast.BinOp object at 0x1051b4dc0>
+; <ast.Constant object at 0x1051b4d60>
   ldi 1 RC
   spu RC
   spo RD
   spo RC
   add RC RD RC
   spu RC
-; assigning value to stack segment + 2
-  ldi vt_stack_addr RA
-  ldr RA RA
+; assigning stack address (FP + 2) := top of stack
+  spo RA
   ldi 2 RB
-  add RA RB RA
-  spo RB
-  str RB RA
-  jpi While_test_2
-@label while_else_3
-@label while_end_4
-; <ast.While object at 0x1048cd0f0>
-@label while_test_8
-; <ast.Constant object at 0x1048cd060>
+  add RB FP RB
+  str RA RB
+  jpi While_test_1
+@label while_else_2
+@label while_end_3
+; <ast.While object at 0x1051b4d30>
+@label while_test_7
+; <ast.Constant object at 0x1051b4d00>
   ldi 1 RB
   spu RB
   spo RA
   addi RA 0 RA
-  bri zero While_else_9
-; <ast.Pass object at 0x1048ccfa0>
-  jpi While_test_8
-@label while_else_9
-@label while_end_10
-  subi SP 3 SP
+  bri zero While_else_8
+; <ast.Pass object at 0x1051b4cd0>
+  jpi While_test_7
+@label while_else_8
+@label while_end_9
+  mov FP SP
+  hlt
diff --git a/asm/test_py_src.atk16_optimized b/asm/test_py_src.atk16_optimized
deleted file mode 100644
index f526853..0000000
--- a/asm/test_py_src.atk16_optimized
+++ /dev/null
@@ -1,202 +0,0 @@
-@use ext_std:*
-@let sp RH
-@let vector_table 0x10
-@let vt_ISR0 0x10
-@let vt_ISR1 0x11
-@let vt_ISR2 0x12
-@let vt_ISR3 0x13
-@let vt_stack_addr 0x14
-@let vt_term_pp_addr 0x15
-@let vt_kb_pp_addr 0x16
-@let vt_gr_mode_addr 0x17
-@let vt_sprite_mem 0x18
-@let vt_text_mem 0x19
-@let stack_segment 0x8000
-@let mmio_segment 0xE000
-@let terminal_addr 0xE000
-@let keyboard_addr 0xE001
-@let gr_mode_addr 0xE002
-@let sprite_mem 0xE800
-@let text_mem 0xF800
-@let gr_disabled_mode 0b00
-@let gr_text_mode 0b01
-@let gr_sprite_mode 0b10
-@address 0x0
-  ldi vt_stack_addr RA
-  ldr RA SP
-  jpi program_segment
-@address vector_table
-  keyboard_isr
-  hlt_isr
-  hlt_isr
-  hlt_isr
-  stack_segment
-  terminal_addr
-  keyboard_addr
-  gr_mode_addr
-  sprite_mem
-  text_mem
-@label hlt_isr
-  ldi 0x55 RE
-  hlt
-@label keyboard_isr
-  spu RA
-  spu RB
-  ldi vt_kb_pp_addr RA
-  ldr RA RA
-  ldr RA RA
-  ldi vt_term_pp_addr RB
-  ldr RB RB
-  str RA RB
-  spo RB
-  spo RA
-  rti
-@label program_segment
-  ldi vt_gr_mode_addr RA
-  ldr RA RA
-  ldi gr_disabled_mode RB
-  str RB RA
-  jpi main
-@label text_mode
-  1
-@label graphics_mode_addr_p
-  23
-@label text_mem_addr_p
-  25
-@label int_1
-  65
-@label int_5
-  26
-@label nth_letter
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 4 RB
-  add RA RB RA
-  ldr RA RA
-  ldi int_1 RB
-  ldr RB RB
-  add RA RB RA
-  spu RA
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 3 RB
-  add RA RB RA
-  spo RB
-  mov RA SP
-  ldr RA RA
-  spu RB
-  jpr RA
-@label main
-  addi SP 3 SP
-  ldi graphics_mode_addr_p RC
-  ldr RC RC
-  ldr RC RC
-  spu RC
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 0 RB
-  add RA RB RA
-  spo RB
-  str RB RA
-  ldi text_mem_addr_p RC
-  ldr RC RC
-  ldr RC RC
-  spu RC
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 1 RB
-  add RA RB RA
-  spo RB
-  str RB RA
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 0 RB
-  add RA RB RA
-  ldr RA RA
-  ldi text_mode RB
-  ldr RB RB
-  str RB RA
-  ldi 0 RC
-  spu RC
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 2 RB
-  add RA RB RA
-  spo RB
-  str RB RA
-  ldi 0b111111111 RG
-@label while_test_2
-  ldi vt_stack_addr RB
-  ldr RB RB
-  ldi 2 RC
-  add RB RC RB
-  ldr RB RB
-  ldi int_5 RC
-  ldr RC RC
-  sub RB RC RB
-  bri carry Compare_true_6
-  ldi 0 RB
-  spu RB
-  jpi Compare_end_7
-@label compare_true_6
-  ldi 1 RB
-  spu RB
-@label compare_end_7
-  spo RA
-  addi RA 0 RA
-  bri zero While_else_3
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 1 RB
-  add RA RB RA
-  ldr RA RA
-  spu RA
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 2 RB
-  add RA RB RA
-  ldr RA RB
-  spo RA
-  add RA RB RA
-  spu RA
-  ldi 0 RA
-  spu RA
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 2 RB
-  add RA RB RA
-  ldr RA RA
-  spu RA
-  subi SP 2 RA
-  lpc RB
-  addi RB 3 RB
-  str RB RA
-  jpi nth_letter
-  spo RB
-  spo RA
-  str RB RA
-  ldi vt_stack_addr RC
-  ldr RC RC
-  ldi 2 RD
-  add RC RD RC
-  ldr RC RC
-  ldi 1 RD
-  add RC RD RC
-  spu RC
-  ldi vt_stack_addr RA
-  ldr RA RA
-  ldi 2 RB
-  add RA RB RA
-  spo RB
-  str RB RA
-  jpi While_test_2
-@label while_else_3
-@label while_end_4
-@label while_test_8
-  ldi 1 RA
-  addi RA 0 RA
-  bri zero While_else_9
-  jpi While_test_8
-@label while_else_9
-@label while_end_10
-  subi SP 3 SP
\ No newline at end of file
diff --git a/asm/test_py_src.py b/asm/test_py_src.py
index 74ea60d..ec17ddc 100644
--- a/asm/test_py_src.py
+++ b/asm/test_py_src.py
@@ -1,6 +1,5 @@
 import atk16
 
-
 TEXT_MODE: atk16.ConstInt            = 1
 GRAPHICS_MODE_ADDR_P: atk16.ConstInt = 0x17
 TEXT_MEM_ADDR_P: atk16.ConstInt      = 0x19
@@ -10,18 +9,12 @@ text_mem_addr = atk16.load(TEXT_MEM_ADDR_P)
 
 atk16.store(graphics_mode_addr, TEXT_MODE)
 
-
 def nth_letter(n: int) -> int:
-    return n + 65
+    return atk16.ord('A') + n
 
 i = 0
-atk16.asm("ldi 0b111111111 RG")
 while i < 26:
-    # bug: stack frame offset for nth_letter is calculated
-    # one off because text_mem_addr + i takes up one
-    # stack slot
     atk16.store(text_mem_addr + i, nth_letter(i))
-    #atk16.store(text_mem_addr + i, i + 65)
     i = i + 1
 
 while True:
diff --git a/digital/atk16.dig b/digital/atk16.dig
index c028bd7..87f7ddb 100644
--- a/digital/atk16.dig
+++ b/digital/atk16.dig
@@ -413,20 +413,6 @@
       <pos x="-140" y="180"/>
     </visualElement>
     <visualElement>
-      <elementName>Tunnel</elementName>
-      <elementAttributes>
-        <entry>
-          <string>rotation</string>
-          <rotation rotation="2"/>
-        </entry>
-        <entry>
-          <string>NetName</string>
-          <string>US_RS</string>
-        </entry>
-      </elementAttributes>
-      <pos x="-280" y="220"/>
-    </visualElement>
-    <visualElement>
       <elementName>VDD</elementName>
       <elementAttributes>
         <entry>
@@ -456,21 +442,6 @@
       <pos x="-240" y="200"/>
     </visualElement>
     <visualElement>
-      <elementName>Reset</elementName>
-      <elementAttributes>
-        <entry>
-          <string>invertOutput</string>
-          <boolean>false</boolean>
-        </entry>
-      </elementAttributes>
-      <pos x="-260" y="260"/>
-    </visualElement>
-    <visualElement>
-      <elementName>Or</elementName>
-      <elementAttributes/>
-      <pos x="-220" y="220"/>
-    </visualElement>
-    <visualElement>
       <elementName>Driver</elementName>
       <elementAttributes>
         <entry>
@@ -1741,11 +1712,6 @@
       <pos x="400" y="740"/>
     </visualElement>
     <visualElement>
-      <elementName>Break</elementName>
-      <elementAttributes/>
-      <pos x="880" y="280"/>
-    </visualElement>
-    <visualElement>
       <elementName>Tunnel</elementName>
       <elementAttributes>
         <entry>
@@ -1773,6 +1739,40 @@
       </elementAttributes>
       <pos x="680" y="1020"/>
     </visualElement>
+    <visualElement>
+      <elementName>Tunnel</elementName>
+      <elementAttributes>
+        <entry>
+          <string>rotation</string>
+          <rotation rotation="2"/>
+        </entry>
+        <entry>
+          <string>NetName</string>
+          <string>US_RS</string>
+        </entry>
+      </elementAttributes>
+      <pos x="-360" y="240"/>
+    </visualElement>
+    <visualElement>
+      <elementName>Reset</elementName>
+      <elementAttributes>
+        <entry>
+          <string>invertOutput</string>
+          <boolean>false</boolean>
+        </entry>
+      </elementAttributes>
+      <pos x="-340" y="280"/>
+    </visualElement>
+    <visualElement>
+      <elementName>Or</elementName>
+      <elementAttributes/>
+      <pos x="-300" y="240"/>
+    </visualElement>
+    <visualElement>
+      <elementName>Break</elementName>
+      <elementAttributes/>
+      <pos x="-200" y="260"/>
+    </visualElement>
   </visualElements>
   <wires>
     <wire>
@@ -1808,10 +1808,6 @@
       <p2 x="960" y="640"/>
     </wire>
     <wire>
-      <p1 x="-260" y="260"/>
-      <p2 x="-220" y="260"/>
-    </wire>
-    <wire>
       <p1 x="420" y="260"/>
       <p2 x="440" y="260"/>
     </wire>
@@ -1828,6 +1824,14 @@
       <p2 x="280" y="260"/>
     </wire>
     <wire>
+      <p1 x="-240" y="260"/>
+      <p2 x="-220" y="260"/>
+    </wire>
+    <wire>
+      <p1 x="-220" y="260"/>
+      <p2 x="-200" y="260"/>
+    </wire>
+    <wire>
       <p1 x="680" y="900"/>
       <p2 x="700" y="900"/>
     </wire>
@@ -1960,8 +1964,8 @@
       <p2 x="700" y="280"/>
     </wire>
     <wire>
-      <p1 x="860" y="280"/>
-      <p2 x="880" y="280"/>
+      <p1 x="-340" y="280"/>
+      <p2 x="-300" y="280"/>
     </wire>
     <wire>
       <p1 x="680" y="920"/>
@@ -2348,14 +2352,6 @@
       <p2 x="740" y="-220"/>
     </wire>
     <wire>
-      <p1 x="-160" y="220"/>
-      <p2 x="-140" y="220"/>
-    </wire>
-    <wire>
-      <p1 x="-280" y="220"/>
-      <p2 x="-220" y="220"/>
-    </wire>
-    <wire>
       <p1 x="60" y="220"/>
       <p2 x="80" y="220"/>
     </wire>
@@ -2372,6 +2368,10 @@
       <p2 x="860" y="220"/>
     </wire>
     <wire>
+      <p1 x="-220" y="220"/>
+      <p2 x="-140" y="220"/>
+    </wire>
+    <wire>
       <p1 x="680" y="220"/>
       <p2 x="760" y="220"/>
     </wire>
@@ -2496,6 +2496,10 @@
       <p2 x="280" y="240"/>
     </wire>
     <wire>
+      <p1 x="-360" y="240"/>
+      <p2 x="-300" y="240"/>
+    </wire>
+    <wire>
       <p1 x="-20" y="240"/>
       <p2 x="80" y="240"/>
     </wire>
@@ -2632,12 +2636,8 @@
       <p2 x="280" y="-100"/>
     </wire>
     <wire>
-      <p1 x="860" y="220"/>
-      <p2 x="860" y="280"/>
-    </wire>
-    <wire>
-      <p1 x="-160" y="220"/>
-      <p2 x="-160" y="240"/>
+      <p1 x="-220" y="220"/>
+      <p2 x="-220" y="260"/>
     </wire>
     <wire>
       <p1 x="480" y="0"/>
diff --git a/digital/atk16_mem.dig b/digital/atk16_mem.dig
index 629a6af..5f5e2e6 100644
--- a/digital/atk16_mem.dig
+++ b/digital/atk16_mem.dig
@@ -34,7 +34,7 @@
         </entry>
         <entry>
           <string>Data</string>
-          <data>4014,2e00,6028,8000,8000,8000,8000,8000,8000,8000,8000,8000,8000,8000,8000,8000,001c,001a,001a,001a,8000,e000,e001,e002,e800,f800,4855,f000,31c0,1fc8,31c8,1fc8,4016,2000,2000,4215,2240,3040,1fc9,23c0,1fc9,21c0,e000,4017,2000,4200,3008,601a,0001,0017,0019,0041,001a,4014,2000,4204,0008,2000,4233,2240,0008,31c0,1fc8,4014,2000,4203,0008,1fc9,23c0,1e00,2000,31c8,1fc8,5000,1fd8,4431,2480,2480,31d0,1fc8,4014,2000,4200,0008,1fc9,23c0,3008,4432,2480,2480,31d0,1fc8,4014,2000,4201,0008,1fc9,23c0,3008,4014,2000,4200,0008,2000,4230,2240,3008,4400,31d0,1fc8,4014,2000,4202,0008,1fc9,23c0,3008,4dff,4214,2240,4402,0250,2240,4434,2480,0251,8004,4200,31c8,1fc8,6003,4201,31c8,1fc8,1fc9,21c0,1000,8436,4014,2000,4201,0008,2000,31c0,1fc8,4014,2000,4202,0008,2200,1fc9,21c0,0008,31c0,1fc8,4000,31c0,1fc8,4014,2000,4202,0008,2000,31c0,1fc8,11d1,9200,1258,3008,618b,1fc9,23c0,1fc9,21c0,3008,4414,2480,4602,0498,2480,4601,0498,31d0,1fc8,4014,2000,4202,0008,1fc9,23c0,3008,61b6,4001,1000,8401,61fc,1fd9</data>
+          <data>4014,2e00,1dc0,6027,8000,8000,8000,8000,8000,8000,8000,8000,8000,8000,8000,8000,001c,001a,001a,001a,8000,e000,e001,e002,e800,f800,4855,f000,31c0,1fc8,31c8,1fc8,4016,2000,2000,4215,2240,3040,1fc9,23c0,1fc9,21c0,e000,4017,2000,4200,3008,6018,0001,0017,0019,0041,001a,4433,2480,31d0,1fc8,4400,0590,2480,31d0,1fc8,1fc9,27c0,1fc9,25c0,0498,31d0,1fc8,1d89,2380,5040,1fd8,4431,2480,31d0,1fc8,1fc9,25c0,2480,31d0,1fc8,1fc9,21c0,4200,0270,3040,4432,2480,31d0,1fc8,1fc9,25c0,2480,31d0,1fc8,1fc9,21c0,4201,0270,3040,4000,0180,2000,31c0,1fc8,4030,2000,31c0,1fc8,1fc9,23c0,1fc9,21c0,3008,4000,31c0,1fc8,1fc9,21c0,4400,31d0,1fc8,1fc9,21c0,4202,0270,3040,4202,0388,2240,31c8,1fc8,4234,2240,31c8,1fc8,1fc9,25c0,1fc9,23c0,0251,8004,4200,31c8,1fc8,6003,4201,31c8,1fc8,1fc9,21c0,1000,8447,4001,0180,2000,31c0,1fc8,4002,0180,2000,31c0,1fc8,1fc9,23c0,1fc9,21c0,0008,31c0,1fc8,31f0,1fc8,4000,31c0,1fc8,4002,0180,2000,31c0,1fc8,1dd1,9000,1020,3180,1d88,617a,1fc9,21c0,1f80,1fc9,2dc0,31c0,1fc8,1fc9,23c0,1fc9,21c0,3008,4000,31c0,1fc8,1fc9,21c0,4402,0590,2480,31d0,1fc8,4401,31d0,1fc8,1fc9,27c0,1fc9,25c0,0498,31d0,1fc8,1fc9,21c0,4202,0270,3040,619f,4201,31c8,1fc8,1fc9,21c0,1000,8401,61f8,1f80,f000</data>
         </entry>
       </elementAttributes>
       <pos x="740" y="100" />
diff --git a/src/ast_compiler.py b/src/ast_compiler.py
index d3fd70e..4dfcf8e 100644
--- a/src/ast_compiler.py
+++ b/src/ast_compiler.py
@@ -28,8 +28,11 @@ class Reg():
     return f"R{self.reg}"
 
 ALL_REGS: list[RegChar] = ["A", "B", "C", "D", "E", "F", "G", "H"]
-STACK_POINTER_REG = "H"
-SPECIAL_REGS: list[RegChar] = [STACK_POINTER_REG]
+FRAME_POINTER_REG_CHAR: RegChar = "G" # points to base of stack frame
+FRAME_POINTER_REG = Reg(FRAME_POINTER_REG_CHAR)
+STACK_POINTER_REG_CHAR: RegChar = "H"
+STACK_POINTER_REG = Reg(STACK_POINTER_REG_CHAR)
+SPECIAL_REGS: list[RegChar] = [FRAME_POINTER_REG_CHAR, STACK_POINTER_REG_CHAR]
 GENERIC_REGS: OrderedDict[RegChar, None] = OrderedDict()
 for char in ALL_REGS:
   if char not in SPECIAL_REGS:
@@ -41,38 +44,6 @@ def format_asm_row(asm: str) -> str:
   else:
     return asm
 
-@dataclass
-class Frame:
-  names: list[str]
-  bindings: dict[str, Label | StackOffset]
-
-  def size(self) -> int:
-    return len(self.names)
-
-class FrameStack:
-  def __init__(self, stack: list[Frame] = []):
-    self.stack: list[Frame] = stack
-
-  def push(self, frame: Frame):
-    self.stack.append(frame)
-
-  def pop(self) -> Frame | None:
-    if len(self.stack) == 0: return None
-    return self.stack.pop()
-
-  def peek(self) -> Frame:
-    return self.stack[self.size() - 1]
-
-  def size(self) -> int:
-    return len(self.stack)
-
-  def total_offset(self) -> int:
-    total: int = 0
-    for frame in self.stack:
-      total += frame.size()
-
-    return total
-
 class Compiler(ast.NodeVisitor):
   def __init__(self):
     self.const_asm: list[str] = []
@@ -82,8 +53,8 @@ class Compiler(ast.NodeVisitor):
     self.function_def_asms: list[str] = []
     self.currently_emitting_asm_list = self.program_asm
 
-    self.frame_stack = FrameStack()
-    self.tmp_stack_values_n: int = 0
+    self.local_bindings: dict[str, StackOffset] = {}
+    self.const_bindings: dict[str, Label] = {}
 
     self.unique_name_counter = 0
     self.latest_break_target: Label | None = None
@@ -102,8 +73,7 @@ class Compiler(ast.NodeVisitor):
     self.emit_label(name)
     self.emit(f"{value}")
 
-    frame = self.frame_stack.peek()
-    frame.bindings[name] = name
+    self.const_bindings[name] = name
 
     self.currently_emitting_asm_list = prev_currently_emitting_asm_list
 
@@ -113,14 +83,6 @@ class Compiler(ast.NodeVisitor):
 
     self.currently_emitting_asm_list.append(asm)
 
-  def emit_stack_push(self, reg: str):
-    self.tmp_stack_values_n += 1
-    self.emit(f"spu {reg}")
-
-  def emit_stack_pop(self, reg: str):
-    self.tmp_stack_values_n -= 1
-    self.emit(f"spo {reg}")
-
   def emit_label(self, label_name: str):
     label_name = label_name.lower()
     self.emit(f"@label {label_name}")
@@ -176,6 +138,11 @@ class Compiler(ast.NodeVisitor):
           case [ast.Constant(str(value))]:
             self.emit(value)
 
+             # return None
+            with self.allocated_reg() as arg:
+              self.emit(f"ldi 0 {arg}")
+              self.emit(f"spu {arg}")
+
           case other: raise Exception(f"asm: invalid args: {other}")
 
       case "store":
@@ -191,12 +158,17 @@ class Compiler(ast.NodeVisitor):
           self.emit(f"spo {arg1}")
           self.emit(f"str {arg2} {arg1}")
 
+          # return None
+          self.emit(f"ldi 0 {arg1}")
+          self.emit(f"spu {arg1}")
+
       case "load":
         if len(args) != 1:
           raise Exception("Invalid number of arguments to load: " + str(len(args)))
 
         self.visit(args[0])
 
+        self.emit("; (load) dereferencing top of stack")
         with self.allocated_reg() as arg:
           self.emit(f"spo {arg}")
           self.emit(f"ldr {arg} {arg}")
@@ -219,7 +191,7 @@ class Compiler(ast.NodeVisitor):
         name = self.get_unique_name("int")
         self.assign_const(name, value)
         self.emit(f"ldi {name} {reg}")
-        self.emit(f"ldr {reg} {reg}")
+        self.emit(f"ldr {reg} {reg} ; {name} = {value}")
 
       self.emit(f"spu {reg}")
 
@@ -228,20 +200,11 @@ class Compiler(ast.NodeVisitor):
     frame_names = self.collect_local_variables(stmts)
 
     self.emit("; stack frame with offsets")
-
-    frame_bindings: dict[str, Label | StackOffset] = {}
-    for idx, name in enumerate(frame_names):
-      offset = self.frame_stack.total_offset() + idx
-      frame_bindings[name] = offset
-
+    for offset, name in enumerate(frame_names):
+      self.local_bindings[name] = offset
       self.emit(f"; {name} {offset}")
 
-    frame = Frame(
-      names = frame_names,
-      bindings = frame_bindings,
-    )
-    self.frame_stack.push(frame)
-    self.emit(f"addi SP {len(frame.names)} SP")
+    self.emit(f"addi SP {len(frame_names)} SP")
 
     for stmt in stmts:
       if type(stmt) == ast.Expr and type(stmt.value) != ast.Call:
@@ -250,16 +213,20 @@ class Compiler(ast.NodeVisitor):
 
       self.visit(stmt)
 
-    self.frame_stack.pop()
-    self.emit(f"subi SP {len(frame.names)} SP")
+    self.emit(f"mov FP SP")
+    self.emit("hlt")
 
   def visit_Expr(self, expr: ast.Expr):
-    # if self.frame_stack.size == 1 and type(expr.value) != ast.Call:
+    # if type(expr.value) != ast.Call:
     #   self.emit("; NOP top-level expression")
     #   return
 
     self.visit(expr.value)
 
+    with self.allocated_reg() as reg:
+      self.emit("; popping free-standing Expr result from stack")
+      self.emit(f"spo {reg}")
+
   def visit_UnaryOp(self, node: ast.UnaryOp):
     self.emit(f"; {node.op}")
     self.visit(node.operand)
@@ -410,6 +377,8 @@ class Compiler(ast.NodeVisitor):
     self.emit_label(label_end)
 
   def visit_Compare(self, node: ast.Compare):
+    self.emit(f"; {node}")
+
     if len(node.ops) > 1:
       raise Exception("Multiple compare ops not supported")
 
@@ -420,12 +389,13 @@ class Compiler(ast.NodeVisitor):
     left = node.left
     right = node.comparators[0]
 
-    self.visit(left)
-    self.visit(right)
-
     label_true = self.get_unique_name("Compare_true")
     label_end = self.get_unique_name("Compare_end")
 
+    self.visit(left)
+    self.visit(right)
+
+    self.emit(f"; Comparing {left} {op} {right}")
     with self.allocated_reg() as reg_lhs, self.allocated_reg() as reg_rhs:
       match op:
         case ast.Lt(): # lhs < rhs
@@ -539,67 +509,42 @@ class Compiler(ast.NodeVisitor):
       raise Exception("Only simple positional args are supported for now in function definitions.")
 
     self.emit_label(fn_name)
-    return_address_name = self.get_unique_name("return_address")
     local_var_names = self.collect_local_variables(fn_stmts)
-    frame_names = [return_address_name] + fn_params + local_var_names
+    frame_names = fn_params + local_var_names
 
     # Move stack pointer to accommodate local variables
     if len(local_var_names) > 0:
       self.emit(f"addi SP {len(local_var_names)} SP")
 
+    prev_local_bindings = self.local_bindings
+    self.local_bindings = {}
     self.emit("; stack frame with offsets")
-    frame_bindings: dict[str, Label | StackOffset] = {}
-    for idx, name in enumerate(frame_names):
-      offset = self.frame_stack.total_offset() + idx
-      frame_bindings[name] = offset
-
+    for offset, name in enumerate(frame_names):
+      self.local_bindings[name] = offset
       self.emit(f"; {name} {offset}")
 
-    frame = Frame(
-      names = frame_names,
-      bindings = frame_bindings
-    )
-    self.frame_stack.push(frame)
-
     for stmt in fn_stmts:
       self.visit(stmt)
 
-    self.frame_stack.pop()
+    self.local_bindings = prev_local_bindings
     self.currently_emitting_asm_list = prev_currently_emitting_asm_list
 
   def visit_Return(self, node: ast.Return):
     self.emit(f"; {node}")
 
-    if self.frame_stack.size() <= 1:
-      raise Exception("Encountered return in the top level stack frame")
-
-    frame = self.frame_stack.peek()
-
-    ret = node.value
-    if ret is not None:
-        self.visit(ret)
-
     with self.allocated_reg() as reg1, self.allocated_reg() as reg2:
-      ret_addr_offset = self.frame_stack.total_offset() - frame.size()
-      self.emit(f"ldi vt_stack_addr {reg1}") # vector table address of stack segment address
-      self.emit(f"ldr {reg1} {reg1}") # stack segment address
-      self.emit(f"ldi {ret_addr_offset} {reg2}") # stack offset
-      self.emit(f"add {reg1} {reg2} {reg1}") # stack address of return address = stack segment start + stack offset
-      self.emit(f"; stack address of return address is stored in reg {reg1}")
-
-      if ret is not None:
-        self.emit(f"spo {reg2}")
+      if node.value is not None:
+        self.visit(node.value)
       else:
-        self.emit(f"ldi 0 {reg2}")
-
-      self.emit(f"; return value is stored in reg {reg2}")
+        self.emit(f"ldi 0 {reg1}")
+        self.emit(f"spu {reg1}")
 
-      self.emit(f"mov {reg1} SP") # reset stack pointer to return address position (i.e. start of stack frame)
-      self.emit(f"ldr {reg1} {reg1}") # return address
+      self.emit(f"; return value is on top of stack")
 
-      self.emit(f"spu {reg2}")
       self.emit(f"; return from function")
-      self.emit(f"jpr {reg1}")
+      self.emit(f"subi FP 1 FP")
+      self.emit(f"ldr FP {reg2}")
+      self.emit(f"jpr {reg2}")
 
   def visit_Call(self, node: ast.Call):
     self.emit(f"; {node}")
@@ -610,7 +555,12 @@ class Compiler(ast.NodeVisitor):
       case ast.Name(name):
         self.emit(f"; Call function {name}")
 
+        # Push the current FP
+        self.emit("spu FP")
+        #self.emit("mov SP FP") # BUG: FP needs to be moved AFTER EVALING ARGS! args depend on FP in calling frame if there are names to resolve!
+
         # Reserve a stack slot for the return address
+        # FP will point to this slot
         with self.allocated_reg() as reg:
           self.emit(f"ldi 0 {reg}")
           self.emit(f"spu {reg}")
@@ -619,30 +569,31 @@ class Compiler(ast.NodeVisitor):
         for arg in node.args:
           self.visit(arg)
 
-        with self.allocated_reg() as reg1, self.allocated_reg() as reg2:
+        self.emit(f"subi SP {1 + len(node.args)} FP")
+
+        with self.allocated_reg() as reg:
           self.emit("; set up return address and jump to subroutine")
-          self.emit(f"subi SP {len(node.args) + 1} {reg1}")
-          self.emit(f"lpc {reg2}")
-          self.emit(f"addi {reg2} 3 {reg2}")
-          self.emit(f"str {reg2} {reg1}")
+          self.emit(f"lpc {reg}")
+          self.emit(f"addi {reg} 4 {reg}") # imm must equal number of primitive instrs from lpc until jpi
+          self.emit(f"str {reg} FP")
+          self.emit(f"addi FP 1 FP")
           self.emit(f"jpi {name}")
+          # ...after return from call...
+          self.emit(f"spo {reg}")
+
+          self.emit("mov FP SP") # move stack pointer to base of stack frame, i.e. top of previous frame
+          self.emit("spo FP") # restore FP of previous frame
+          self.emit(f"spu {reg}")
 
       case other:
         raise NotImplementedError(f"Unhandled Call: {other}")
 
   def resolve_name(self, name: str) -> Label | StackOffset:
-    frames = FrameStack(self.frame_stack.stack.copy())
+    if name in self.local_bindings:
+      return self.local_bindings[name]
 
-    while frames.size() > 0:
-      frame = cast(Frame, frames.pop())
-
-      if name in frame.bindings:
-        addr = frame.bindings[name]
-        match addr:
-          case Label(value):
-            return value
-          case StackOffset(value):
-            return value
+    if name in self.const_bindings:
+      return self.const_bindings[name]
 
     raise Exception(f"{name} is unbound")
 
@@ -652,20 +603,18 @@ class Compiler(ast.NodeVisitor):
     name = node.id.lower()
     addr = self.resolve_name(name)
 
-    with self.allocated_reg() as reg1, self.allocated_reg() as reg2:
+    with self.allocated_reg() as reg:
       match addr:
         case Label(label):
-          self.emit(f"ldi {label} {reg1}")
+          self.emit(f"ldi {label} {reg}")
         case StackOffset(offset):
-          self.emit(f"ldi vt_stack_addr {reg1}")
-          self.emit(f"ldr {reg1} {reg1}")
-          self.emit(f"ldi {offset} {reg2}")
-          self.emit(f"add {reg1} {reg2} {reg1}")
+          self.emit(f"ldi {offset} {reg}")
+          self.emit(f"add FP {reg} {reg}")
         case other:
           raise Exception(f"Unsupported addr value: {other}")
 
-      self.emit(f"ldr {reg1} {reg1}")
-      self.emit(f"spu {reg1}")
+      self.emit(f"ldr {reg} {reg}")
+      self.emit(f"spu {reg}")
 
   def visit_Assign(self, node: ast.Assign):
     self.emit(f"; {node}")
@@ -679,18 +628,17 @@ class Compiler(ast.NodeVisitor):
         if type(offset) != StackOffset:
           raise Exception(f"Invalid address {offset} for name {name}. Can only assign to stack offsets.")
 
-        self.emit(f"; assigning {name} at stack segment + {offset}")
+        self.emit(f"; assigning {name} at FP + {offset}")
 
         with self.allocated_reg() as reg1, self.allocated_reg() as reg2:
           self.emit(f"; evaluating value to be assigned")
           self.visit(value)
-          self.emit(f"; assigning value to stack segment + {offset}")
-          self.emit(f"ldi vt_stack_addr {reg1}")
-          self.emit(f"ldr {reg1} {reg1}")
+          self.emit(f"; assigning stack address (FP + {offset}) := top of stack")
+          self.emit(f"spo {reg1}") # value
+
           self.emit(f"ldi {offset} {reg2}")
-          self.emit(f"add {reg1} {reg2} {reg1}") # address
-          self.emit(f"spo {reg2}") # value
-          self.emit(f"str {reg2} {reg1}")
+          self.emit(f"add {reg2} FP {reg2}") # address
+          self.emit(f"str {reg1} {reg2}")
 
       case other:
         raise Exception(f"Unsupported assign targets: {other}")
@@ -728,10 +676,19 @@ class Optimizer:
     asm = [row for row in asm if not len(row) == 0]
     asm = [tokenize(row, retain_curlies=True) for row in asm]
     asm = self.compact_spu_spo_pattern(asm)
-    asm = self.compact_target_mov_pattern(asm)
-    asm = self.compact_target_mov_pattern(asm)
-    asm = self.compact_mov_source_pattern(asm)
-    asm = self.compact_mov_source_pattern(asm)
+
+    # TODO: not safe when setting loading SP and FP
+    #   ldr RA SP
+    #   mov RA FP
+    # gets optimized to
+    #   ldr RA FP
+    #asm = self.compact_target_mov_pattern(asm)
+    #asm = self.compact_target_mov_pattern(asm)
+
+    # TODO: not safe at all. E.g. breaks a while True: pass loop
+    #asm = self.compact_mov_source_pattern(asm)
+    #asm = self.compact_mov_source_pattern(asm)
+
     asm = self.compact_spu_load_spo_pattern(asm)
     # asm = self.convert_alr_to_ali(asm)
     result = "\n".join([format_asm_row(" ".join(row)) for row in asm])
@@ -758,15 +715,27 @@ class Optimizer:
         continue
 
       if current[0] == "spu" and next[0] == "spo":
+        print("=== compact_spu_spo_pattern")
+        print(current)
+        print(next)
+
         arg_current = current[1]
         arg_next = next[1]
 
+        print("=== arg_current:", arg_current)
+        print("=== arg_next:", arg_next)
+
         if arg_current == arg_next:
+          print("=== pass")
           pass # remove both spu and spo
         else:
+          print("=== mov")
           mov = ["mov", arg_current, arg_next]
           result.append(mov)
+          print("=== mov:", mov)
 
+        print("=== last of result:", result[len(result) - 1])
+        print()
         i += 1
         continue
 
@@ -890,11 +859,11 @@ asm_out = compiler.compile(
   source_py,
 )
 
-optimizer = Optimizer()
-asm_out_optimized = optimizer.optimize(asm_out)
+# optimizer = Optimizer()
+# asm_out_optimized = optimizer.optimize(asm_out)
 
 with open(outfile_path, "w") as f:
   f.write(asm_out)
 
-with open(f"{outfile_path}_optimized", "w") as f:
-  f.write(asm_out_optimized)
+# with open(f"{outfile_path}_optimized", "w") as f:
+#   f.write(asm_out_optimized)