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

Tests/expected/cosmosBreak.out

@@ -3,11 +3,11 @@ CosmOS
 > two stops, and what the registers were at each
 break at 200E
 A 11 B 22 Q 00 status 00 
-DP0 1030 DP1 0661 DP2 039A DP3 2000 SP FFFF
+DP0 1030 DP1 0663 DP2 039C DP3 2000 SP FFFF
 press a key 
 break at 2023
 A 44 B 55 Q 00 status 00 
-DP0 1000 DP1 0661 DP2 039A DP3 2000 SP FFF5
+DP0 1000 DP1 0663 DP2 039C DP3 2000 SP FFF5
 press a key 
 carried on to the end
 finished

Tests/expected/cosmosStream.out

@@ -0,0 +1,16 @@
+CosmOS
+> loaded, starting at 2000
+> big.txt is 329 blocks
+read 329 blocks, checksum 57368, stopped with 3
+small.txt streamed is 15216, read whole is 15216
+the same
+the same block twice with another file between: the same
+renamed small.txt
+the old name now answers 2
+the new name answers 0
+a name that was never there answers 2
+a block past the end answers 3
+finished
+> halted
+Execution halted.
+[exit 0]