Moving toward new blockloader code

pyasm/pyasm

@@ -75,6 +75,9 @@ BlockBufferBase = None     # base address of the block
 # mask for 16bit values
 WordMask = 0xFFFF
 
+# high bit in 16-bit word
+HighBit = 0x8000
+
 # the output PTP filename extension
 PTPExtension = '.ptp'
 
@@ -237,7 +240,7 @@ BlockLoader = [
               #        ; integers, incrementing the sum whenever the 16bit value overflows.
               #        ;
               #        ; The end of the load is signalled by a block with a
-              #        ; starting address 0177777.
+              #        ; starting address WordMask.
               #        ;
               #        ; Disassembled from the 40tp_simpleDisplay.ptp image file.
               #        ;
@@ -267,7 +270,7 @@ BlockLoader = [
      0102001, # 003725         asn             ;
      0013746, # 003726         jmp     newblk  ;if same, get next block
      0000000, # 003727         hlt             ;if not same, ERROR
-     0177777, # 003730 neg1    data    0177777 ;
+     WordMask, # 003730 neg1    data    WordMask ;
               #        ;------------------------
               #        ;Compute checksum.  Word to sum in AC.
               #        ;------------------------
@@ -367,17 +370,18 @@ def write_word(word):
     write_byte(word >> 8)
     write_byte(word)
 
-def write_endload():
-    """Write the special 'end of load' block."""
+def write_start(address, ac=None):
+    """Write the start block.
 
-    write_byte(1)
-    write_word(0xffff)
+    Add the AC contents word if passed.
+    """
 
-def write_start(address):
-    """Write the start block."""
+    if ac is None:
+        ac = 0
 
-    start_block(address)
-    write_block()
+    write_word(address + HighBit)
+    if address != 0 and ac is not None:
+        write_word(ac & WordMask)
 
 def write_leader(size=ZeroLeaderSize):
     """Write the papertape leader."""
@@ -393,8 +397,6 @@ def write_block_loader():
     for word in BlockLoader:
         write_word(word)
 
-    write_leader()
-
 def start_block(addr):
     """Prepare next block to start at 'addr'"""
 
@@ -426,15 +428,17 @@ def write_block():
         # block buffer is empty, do nothing
         return
 
-    # emit the block size and load address
-    write_byte(code_block_size)
+    # emit the block size and data word count (negated)
     write_word(BlockBufferBase)
-    for word in BlockBuffer:
-        write_word(word)
+    write_word((~code_block_size+1) & WordMask)
 
     # calculate and write the checksum
-    checksum = sum(BlockBuffer) & WordMask
-    write_word(checksum)
+    checksum = (BlockBufferBase + ((~code_block_size+1) & WordMask) + sum(BlockBuffer))
+    write_word((~checksum+1) & WordMask)
+
+    # finally, write out data words
+    for word in BlockBuffer:
+        write_word(word)
 
     # reset the code buffer
     start_block(None)
@@ -774,9 +778,8 @@ def pass_2(lines):
 
     # write the final block of code and optional start address
     write_block()
-#    if StartAddress is not None:
-#        write_start(StartAddress)
-    write_endload()
+    if StartAddress is not None:
+        write_start(StartAddress, ac=0177777)
     write_leader()
 
     # check nothing after END