Load a program that has no data

A five instruction program that writes one port and exits has no Data Segment at
all, and the loader stopped the machine dead on it. It asked the memory
controller to move a segment of no bytes, and a length of zero asks for the
whole 64K - which is the machine's rule, and a reasonable one, since two bytes
cannot say 65536 and a transfer of nothing is not usually what anybody meant. It
is exactly what was meant here. 64K did not fit, the controller refused, and the
load stopped half done.

ON A TERMINAL THAT PRINTS A FAULT WITH AN ADDRESS. Behind a window it is a
frozen picture and no reason at all, which is how it was found and is a separate
problem from this one.

The header says how long each segment is, so the loader knows before it asks.
Both bytes are already in hand, so the test costs one OR. Nothing is lost by
skipping the transfer: a blit leaves the controller's addresses past whatever it
touched, and a blit of nothing would have left them where they already are,
which is where the vectors are read from next.

Guarded for the code segment too. A program with no code is equally assemblable
and would have stopped in exactly the same place.

Mode.sbx is the fix's test and a program worth having on its own: forty columns
or eighty, whichever the screen is not in, which is what a person wanting Snake
drawn twice the size actually needs. Ten instructions and no data, deliberately
- it prints its two digits a register at a time rather than from a string, so it
stays the smallest shape a loadable program can take. Nothing else on that disk
had ever been that shape, which is why nothing had ever tried it.

Reported by the user, who wrote the program.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Programs/CosmOS/Source/cosmos.asm

@@ -1548,11 +1548,29 @@ loadVersionKnown:
   LDA.0
   OUTA 0xE6
   INCD.0
-  LDA.0
-  OUTA 0xE7
+  LDB.0
+  OUTB 0xE7
+
+  ; ---- Nothing to move is not the same as everything to move ----
+  ;
+  ; A LENGTH OF ZERO ASKS THE CONTROLLER FOR THE WHOLE 64K, which is the machine's rule and
+  ; a sensible one for a length somebody typed: two bytes cannot say 65536, and a transfer
+  ; of no bytes is not usually what anybody meant. It is exactly what is meant here. A
+  ; segment can genuinely be empty - a five instruction program that only writes to a port
+  ; has no data at all - and this asked to move 65536 bytes into a bank that has not got
+  ; them, so the controller refused and the machine stopped in the middle of loading.
+  ;
+  ; The header says how long the segment is, so the loader knows before it asks. It has
+  ; both bytes of the length in hand here, so testing them costs one instruction.
+  ;
+  ; NOTHING IS LOST BY SKIPPING IT. A blit leaves the controller's addresses past whatever
+  ; it touched, and a blit of nothing would have left them exactly where they are.
+  OR
+  BRQ loadNoCode
 
   INIA 0x01
   OUTA 0xE8             ; Blit.
+loadNoCode:
 
   ; Then the data. A blit leaves its addresses past whatever it touched, so the source is
   ; already sitting on the first byte of the data and only the destination changes.
@@ -1571,11 +1589,16 @@ loadVersionKnown:
   LDA.0
   OUTA 0xE6
   INCD.0
-  LDA.0
-  OUTA 0xE7
+  LDB.0
+  OUTB 0xE7
+
+  ; And the same for the data, which is the segment that is actually empty in practice.
+  OR
+  BRQ loadNoData
 
   INIA 0x01
   OUTA 0xE8             ; Blit.
+loadNoData:
 
   ; ---- The vectors it brought ----
   ;