Streaming: read a file bigger than the machine's memory

osFileRead hands over a whole file, which settles anything under 64K and
settles nothing above it. CosmOS's own source is above it - the sources
together are 104K against 64K of Data Memory - so a machine that is going
to assemble itself needs another way to ask.

osFileInfo (0d26) says how many blocks a file occupies. osFileBlock (0d27)
hands over one of them and says how many of its bytes belong to the file.
Between them a program reads a file of any size through a buffer of 256.

Blocks rather than bytes from osFileInfo is forced, not chosen: a file on a
sixteen megabyte disk is up to twenty four bits long and a pointer holds
sixteen. osFileBlock's count answers in DP3 for the same kind of reason -
a whole block is 256 bytes, which does not fit in a register, and a count
that reported it as zero would make every reader special-case the end.

Nothing is kept open. Every call names the file, so there is no handle to
leak and nothing left behind by a program that stops halfway. Taken at its
word that means searching the directory once per block, so the system
remembers where the last file it was asked about lives; every path that can
change what a name means calls fileForget, including the shell's own delete
and rename, which do not go through the services. Correctness never depends
on the cache - a cache thrown away is indistinguishable from one never
filled. Measured on a 329 block file: 7% of the run saved when the file is
the first directory entry, 11% when it is the sixteenth.

These two say WHY when the answer is no, which the others do not. Elsewhere
the only useful response to a failure is to give up, so one value suffices.
These are asked questions, and running off the end is how a reader learns it
has finished, so it gets an answer of its own: 1 no disk, 2 no such file,
3 past the end, 4 the disk refused.

Apps/Stream.asm reads an 84,000 byte file through 256 bytes. The check that
matters is the second one: a small file read BOTH ways - whole with
osFileRead and streamed - with the two checksums compared, so streaming is
measured against the path already known to work rather than against a number
someone wrote down. The checksum is Fletcher's rather than a sum, because a
sum is the same whatever order the bytes arrived in and the order is exactly
what streaming has to get right. Both checksums were also confirmed against
the same arithmetic run on the host.

The rest of the test is the cache: two files read alternately catch a memory
that missed the name changing, and a rename catches one that missed the file
moving - and that one would otherwise pass, since the blocks are still there
holding the same bytes.

The test file is generated rather than taken from the repository. The CosmOS
sources would be a truer picture and would move the recorded checksum every
time a line of CosmOS was edited, putting a real difference in a crowd of
meaningless ones - the same trap the cycle counts used to set.

cosmosBreak's recorded output moves by two bytes in two pointers: SbfsIndex
added two bytes to the filesystem's data and Break prints the system
addresses the registers happened to hold.

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

Programs/CosmOS/Source/services.asm

@@ -45,6 +45,35 @@
   osFileDelete   0d22   ; DP0 names it. Q is zero if it went.
   osFileRename   0d23   ; DP0 is the name it has, DP1 the name it should have. Q is zero if it moved.
 
+; ---- Reading a file that will not fit ----
+;
+; osFileRead answers with a whole file in Data Memory, which settles it for anything under
+; 64K and settles nothing above. These two are the other way of asking: how big is it, and
+; then give me one block of it at a time. Nothing is kept between the calls but the number
+; of the block wanted, so there is no handle to open, none to close, and nothing left
+; behind by a program that stops in the middle. The system remembers where the last file it
+; was asked about lives, so asking for four hundred blocks of one file costs one search of
+; the directory rather than four hundred; that is a speed, not a promise, and a caller
+; never has to know about it.
+;
+; THESE TWO SAY WHY WHEN THE ANSWER IS NO, which the others do not. Everywhere else the
+; only useful thing to do about a failure is to give up, so one value is enough. These
+; exist to be asked questions with - is it there, is there any more of it - and the
+; difference between "no disk", "no such file" and "that was the last block" is the answer
+; rather than an excuse.
+;
+;   1  there is no disk
+;   2  there is no file of that name
+;   3  that block is past the end of the file      (osFileBlock only)
+;   4  the disk would not read it                  (osFileBlock only)
+  osFileInfo     0d26   ; DP0 names it. Q is zero if it is there, DP3 is how many blocks.
+  osFileBlock    0d27   ; DP0 names it, DP1 says where, A and B are which block from zero.
+                        ; Q is zero if it read, DP3 is how many of its bytes are the file's:
+                        ; a whole 0d256 except in a last block that is short. That count is
+                        ; why DP3 answers and not a register - 0d256 does not fit in a byte,
+                        ; and a count that lied about a full block would make every reader
+                        ; treat the end of a file as a special case.
+
 ; ---- And with the console ----
 ;
 ; printString is already up there. This is the other half of what a program prints: a