Add override_mode to allow user to write own ISRs to vector table

2 files changed, 43 insertions, 27 deletions

diff --git a/atk16_asm/asm_pass3.py b/atk16_asm/asm_pass3.py
index ec8b20c..a3f9537 100644
--- a/atk16_asm/asm_pass3.py
+++ b/atk16_asm/asm_pass3.py
@@ -50,12 +50,22 @@ def pass_3(result2: Result2) -> Result3:
       original_text=original_text,
     )
 
+  # On address conflict, if override_mode is True, the last definition wins
+  # otherwise, an exception is raised
+  override_mode = False
+
   for line in result2.lines:
     keyword, *args = line.parts
     match keyword:
       case "@address":
         address = eval_expr(symbols, args[0])
         continue
+      case "@begin_override":
+        override_mode = True
+        continue
+      case "@end_override":
+        override_mode = False
+        continue
       case "@label":
         label = args[0]
         if label in symbols:
@@ -67,6 +77,10 @@ def pass_3(result2: Result2) -> Result3:
         symbols[args[0]] = eval_expr(symbols, args[1])
         continue
       case _:
+        if override_mode:
+          # TODO this is a bit inefficient, but works
+          result_lines = list(filter(lambda l: l.address != address, result_lines))
+
         dbg_original_text = " ".join(line.original_parts)
         dbg_expanded_text = " ".join(line.parts)
         save_dbg_source(address, line.src_file, line.line_num, dbg_expanded_text, dbg_original_text)
diff --git a/atk16_asm/builtin_asm/bootstrap.atk16 b/atk16_asm/builtin_asm/bootstrap.atk16
index 9b024e6..94eed4c 100644
--- a/atk16_asm/builtin_asm/bootstrap.atk16
+++ b/atk16_asm/builtin_asm/bootstrap.atk16
@@ -1,18 +1,5 @@
 @let SP              RH
 
-; vector table fields
-@let vector_table    0x10
-@let vt_ISR0         0x10 ; ISR0
-@let vt_ISR1         0x11 ; ISR1
-@let vt_ISR2         0x12 ; ISR2
-@let vt_ISR3         0x13 ; ISR3
-@let vt_stack_addr   0x14 ; Stack address
-@let vt_term_pp_addr 0x15 ; Terminal peripheral address
-@let vt_kb_pp_addr   0x16 ; Keyboard peripheral address
-@let vt_gr_mode_addr 0x17 ; Graphics mode setting address
-@let vt_sprite_mem   0x18 ; Sprite memory address
-@let vt_text_mem     0x19 ; Text memory buffer address
-
 ; memory segments
 @let stack_segment   0x8000
 @let mmio_segment    0xE000
@@ -29,8 +16,21 @@
 @let gr_text_mode     0b01
 @let gr_sprite_mode   0b10
 
+; vector table fields
+@let vector_table    0x10
+@let vt_ISR0         0x10 ; ISR0
+@let vt_ISR1         0x11 ; ISR1
+@let vt_ISR2         0x12 ; ISR2
+@let vt_ISR3         0x13 ; ISR3
+@let vt_stack_addr   0x14 ; Stack address
+@let vt_term_pp_addr 0x15 ; Terminal peripheral address
+@let vt_kb_pp_addr   0x16 ; Keyboard peripheral address
+@let vt_gr_mode_addr 0x17 ; Graphics mode setting address
+@let vt_sprite_mem   0x18 ; Sprite memory address
+@let vt_text_mem     0x19 ; Text memory buffer address
+
 @address vector_table
-  keyboard_isr   ; 0x10
+  hlt_isr        ; 0x10
   hlt_isr        ; 0x11
   hlt_isr        ; 0x12
   hlt_isr        ; 0x13
@@ -45,19 +45,21 @@
   ldi 0x55 RE
   hlt
 
-@label keyboard_isr
-  spu RA
-  spu RB
-  ldi vt_kb_pp_addr RA    ; RA := Keyboard peripheral address pointer
-  ldr RA RA               ; RA := Keyboard peripheral address deref
-  ldr RA RA               ; RA := <16-bit keyboard character code> from MMIO register
-  ldi vt_term_pp_addr RB  ; RB := Terminal peripheral address pointer
-  ldr RB RB               ; RB := Terminal peripheral address deref
-  str RA RB               ; terminal <- character code (ASCII?)
-  mov RA RE               ; RE := character code (for debugging)
-  spo RB
-  spo RA
-  rti
+; Example ISR for keyboard to terminal
+; @label keyboard_to_terminal_isr
+;   spu RA
+;   spu RB
+;   ldi vt_kb_pp_addr RA    ; RA := Keyboard peripheral address pointer
+;   ldr RA RA               ; RA := Keyboard peripheral address deref
+;   ldr RA RA               ; RA := <16-bit keyboard character code> from MMIO register
+;   ldi vt_term_pp_addr RB  ; RB := Terminal peripheral address pointer
+;   ldr RB RB               ; RB := Terminal peripheral address deref
+;   str RA RB               ; terminal <- character code (ASCII?)
+;   mov RA RE               ; RE := character code (for debugging)
+;   spo RB
+;   spo RA
+;   rti
+
 @label program_segment
 
 ; define short prelude that sets up vital instructions