HALT is terminal - stepCPU returns at once when the Halt Flag is up, so a
halted machine does not execute, service devices, or take an interrupt -
and that has to stay true, because every test ends with a halt and "halted"
is how a program says it has finished. The consequence was that SplitBit
had no way to wait at all. Every wait was a spin, and a spin is bus
traffic: 11.5% of Type over a 14K file on a disk of ten thousand cycles,
after read-ahead had already hidden three quarters of the latency.
WAIT is 0xFE, one byte, no operands, sitting under HALT where the
instruction that almost stops the machine belongs. Three decisions in it:
- A line already standing means there is nothing to wait for, so WAIT does
nothing. That is what makes test-then-wait race-free.
- Any line ends the wait, masked or not, so a program can sleep on a device
it has no handler for and read its status afterwards. Masking says who
answers a request, not whether it happened.
- A line that wakes the CPU without being dispatched is taken down by the
WAIT. Left standing it would be found by the next WAIT, which would
return at once - the program would spin exactly as before while looking
as though it slept.
Waiting is NOT a Status bit, and that is the trap avoided rather than a
gap: Status rides into the interrupt frame and comes back out, so a machine
interrupted mid-wait would return from its handler still waiting, and wait
again for what it had already been given. An internal field instead.
Idle cycles are counted apart from bus cycles and the halt line says so
when there are any, which is what makes the difference observable at all -
with the line-clearing removed the total moves by ONE cycle, 20,100 against
20,099, and only the idle half changes, halving to 9,976. A test on
totals could never have seen it. Tests/terminal.sh asks that question,
being the file for things a recorded output cannot see, and fails with the
clear removed while "both reads finished" still passes.
Three collisions, all found by building it:
- 0xFE was the assembler's "not an instruction" sentinel. getOpcode now
answers a negative NOT_AN_OPCODE, which is outside the range of every
possible answer instead of inside the unused part of it.
- 0xFE was also what faultTest and faultResumeTest executed to provoke a
fault. They now use 0xFD and say why, because they did not fail when it
became an instruction - they HUNG, having started sleeping instead.
- Keys.asm has had a label called "wait" for a year, and mnemonics are
matched uppercased. What that reported was "Branch without label" at the
BRQ thirty lines away. The assembler now refuses a label that is already
an instruction, at the label, by name; every instruction added takes a
word out of the space of label names, so this will happen again.
The memory map gave CosmOS 0x0000 through 0x1FFF of Program Memory and
applications 0x2000 and above. CosmOS is 8141 bytes at the previous commit,
which is fifty one bytes short of the line, and the next thing added to it
went over.
GOING OVER DOES NOT FAIL WHERE IT HAPPENS. Nothing enforces the division: an
application says where it goes with #Base and the loader puts it there, so a
CosmOS that has grown past 0x1FFF simply has the next program loaded written
over the end of it. What breaks is whichever part of the shell that program
happened to cover, at whatever later moment somebody uses it. It turned up here
as the monitor's assemble command answering "I do not know" to valid
instructions, several commands into a session, on a machine that had booted
perfectly well.
Both halves are doubled: applications now start at 0x4000 in Program Memory and
0x2000 in Data Memory. That is 16K of code and 8K of data for the system,
against the 8775 and 2948 it uses today. Both were on the same trajectory, and
moving them together means the twenty files that say #Base are edited once
rather than twice.
The standalone loader's loadable.asm keeps its old base: it belongs to the
loader CosmOS grew out of, not to CosmOS, and its addresses answer to a
different program. The unbased-segment diagnostic keeps its old base too - it
exists to produce an error message that names the address, and the message is
what is recorded.
Tests/docs.sh now reads the two limits out of the table in the README and
measures both segments against them. It reads them rather than being told them
because the table is the specification, and this is the second time in this
project that the thing nobody checked is the thing that rotted.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
Anachronaut
