A boot payload can arrange its own Data Segment

Stage one places Program Memory and nothing else, because knowing where a
payload's data ends and its code begins would mean knowing a format, and
knowing formats is what ROM must do as little of as possible. But the real
second stage needs a Data Segment: sbfs.asm has variables and a string it
compares against.

The answer needs nothing new. A loadable image is written into the slot as
code followed by data, so the data image is already in Program Memory just
past the code - and the payload's first instructions blit it down to where
it was assembled for. Proved by slotData.asm, which prints from a string it
placed itself.

The padding is the part worth recording. The blit needs a length and the
assembler will not work out the difference between two labels, so the
segment is padded to a round number and that number is what gets copied.
The first draft padded to 257 and copied 256, and the byte that did not
arrive was padding, so it worked by luck. It is exact now and says why.

This is the shape the user asked for and it goes further than the
mechanism: the second stage becomes a loader for the machine's ORDINARY
image format rather than for anything boot-specific, so bare metal SplitBit
stops being a special case. A program that wants no operating system is
just an image, developed under CosmOS like any other, and selectable at
boot because it is a file.

Programs/Boot/slotData.asm

@@ -0,0 +1,65 @@
+; slotData.asm
+; A boot slot payload that arranges its own Data Segment.
+;
+; Stage one places Program Memory and nothing else, because knowing where a payload's data
+; ends and its code begins would mean knowing a format, and knowing formats is the thing
+; ROM must do as little of as possible. So a payload that wants initialised data arranges
+; it: a loadable image is written into the slot as code followed by data, which leaves the
+; data image sitting in Program Memory just past the code, and the first thing the payload
+; does is blit it down to where it was assembled for.
+;
+; This is how the real second stage will get the Data Segment that sbfs.asm needs.
+;
+; THE PADDING IS NOT DECORATION. The blit needs a length, the assembler will not work out
+; a difference between two labels, so the segment is padded to a round number and that
+; number is what gets copied. Reserve one byte too many and the last byte does not arrive:
+; the first draft of this padded to 257 and copied 256.
+;
+; Written by Anachronaut
+#Program
+  #Base 0xC000
+
+start:
+  RSTA
+  OUTA 0xE0             ; SourceBank: Program Memory, where stage one put everything.
+  SETD.0 codeEnd
+  PSHD.0
+  POPA
+  POPB
+  OUTB 0xE1
+  OUTA 0xE2
+  INIA 0d1
+  OUTA 0xE3             ; DestBank: Data Memory.
+  SETD.0 Greeting
+  PSHD.0
+  POPA
+  POPB
+  OUTB 0xE4
+  OUTA 0xE5
+  INIA 0x01
+  OUTA 0xE6
+  RSTA
+  OUTA 0xE7             ; A round 256 bytes, which the segment is padded to.
+  INIA 0x01
+  OUTA 0xE8
+
+  SETD.0 Greeting
+sayLoop:
+  LDA.0
+  BRA sayDone
+  OUTA 0x00
+  INCD.0
+  BRI sayLoop
+sayDone:
+  HALT
+
+codeEnd:
+
+#Data
+  #Base 0x3000
+
+Greeting:
+"a payload with data of its own
+"
+Padding:
+  #Reserve 0d224