main
7
Commits
| Author | SHA1 | Message | Date | |
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 Anachronaut
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7b28f48f52 |
Once: start something else on the next start, and only that one
A program that owns the whole machine had nowhere to run. It cannot be started from the shell, because starting it means there is no shell, and pointing boot.cfg at it means a machine that keeps starting it - which is a poor place to find a mistake in something written five minutes ago. Once writes /System/Boot/once.cfg, in the same format as boot.cfg and read with the same routines, because a second format for one setting would be a second format. The loader reads it before boot.cfg and DELETES IT BEFORE IT JUMPS, which is the only moment there is: after the jump the loader does not exist. Consumed by being read rather than by working, so a one shot that hangs cannot hang twice - the request is gone before the image ran, and the next start reads boot.cfg like any other. THE BOOT STATE IS NOT TOUCHED, and the first version got that wrong. It marked the start the way any other start is marked, and then every successful bare metal boot reported that it had never arrived - because a program with the whole machine has no filesystem to clear a mark with and is doing nothing wrong by not having one. Found by running it: the image printed its line and the next start still said the last one did not. Three disks, each a start further along, so none of the tests depends on another having run. The loop is closed on the machine now: write it in Edit, assemble it with Asm, ask for it with Once, restart, watch it own the machine, and the system comes back without being asked. |
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Anachronaut
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ce2a2cd7e6 |
Settle is a program, and a machine with no fallback still starts
The boot state opened a loop that could not be closed from inside: the machine said "settle it to try again" and gave you no way to do so. Settle closes it, in 349 bytes. A PROGRAM RATHER THAN A SHELL WORD. The shell is for the things that cannot be done without it, and this is not one - it reaches the system through SWI like anything else, which means it can be replaced, left off a disk, or called by whatever comes to call programs in turn. That last one is the point: a shell word is not callable by anything. Two services for it. osBootState answers in Q, and a machine with no disk answers settled, because there is nothing there to be unsettled about. osBootSettle puts it back. SETTLING IS THE ONLY WRITE A PROGRAM GETS - marking a start as trying or fallen back is the loader's business, and a service that let a program claim either would let it lie about something the loader has no way to check. And a hole the tests walked into, which was mine rather than theirs. With no fallback configured, a failed start left the machine unable to start at all: the mark said do not use the system, and there was nothing else to use. That turns "the last start failed" into "no start is permitted", which is worse than the problem the mark was added to solve. With nothing to fall back to it now tries the configuration again and says so - a failure that was passing recovers, and one that is not leaves the machine exactly where it would have been without any of this, which is the most that can be promised when there is only one thing to start. docs.sh caught both new services having no row in the services table before anything else did. |
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Anachronaut
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dc74149321 |
B4: the disk remembers whether the last start arrived
The loader marks the superblock before it hands over and the system clears the mark when it reaches its prompt, so a system that crashes on the way there leaves it set. The loader finding it still set next time is how a machine that will not start says so to the only thing in a position to do anything about it. Without that, pointing boot.cfg at something that dies before the shell is a machine that can never be told anything again - the shell is the only way to change the file, and the file is what stops the shell from starting. Three states rather than two, and the third is the one worth having: 0 settled the last start arrived; use the configuration 1 trying handed over, and nothing came back to say it got there 2 fell back a try failed and the fallback was used, until settled With only 0 and 1 the machine alternates for ever: fall back, reach a prompt, clear the mark, retry the broken system, crash, fall back. State 2 stops that. A system known not to start is not tried again until somebody says the situation has changed. REACHING THE PROMPT IS A DELIBERATE THRESHOLD. It is not a claim that the system works - a shell can be reached by something broken in every other way. It is the point where a person can type, which is exactly what the fallback exists to give back: anything wrong past there is fixable from the prompt and nothing wrong before it is fixable at all. The routines live in sbfs.asm because both the loader and the system read and write this byte, and two pieces of code with their own idea of where a byte lives is what this format has two implementations and a byte for byte comparison to avoid. And the trap this system documents in its own manual caught me anyway: the first version handed the state back in A, which CALL restores, so every read got whatever the caller happened to be holding. It comes back in memory now, and the comment says why. Three disks differing only in the state on them, so the tests read as three consecutive starts of one machine while none depends on another running. |
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Anachronaut
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546f336823 |
Configuration files, and boot.cfg as the first of them
One setting to a line: a key, a space, the rest of the line is the value. A semicolon starts a comment. The format was noticed rather than designed - textSplit already cuts the first word off a line and leaves the rest, and textSame already insists two strings end together, so reading a setting is those two routines and a loop. It is also what the shell reads, which makes a configuration line a command line the machine reads instead of a person typing one. The format was chosen by asking what the BOOT LOADER could manage, because it is the worst case in every direction: a few kilobytes, no operating system to report to, and if it fails the machine does not start. Two formats would be worse than one and the loader cannot have the richer one. CONFIGURATION IS ADVICE. A missing file, a missing key, an unusable value, a line too long to read: all of them mean use the default and none is a failure. BUT QUIET IS NOT SILENT - a setting somebody meant, which did not take effect, says so. That was the user's addition and it is the better rule: the default alone leaves the only symptom being that the machine did not do what somebody asked. So two routines. cfgGet reads and says nothing, because reading three settings should not report one bad line three times. cfgCheck reads the file once and reports, and is handed the caller's list of keys - whether a key means anything is the only part of this a shared reader cannot judge. /System/Boot/ holds the boot files, and stage two reads boot.cfg for what to start, with a fallback to try if it does not work and a name compiled in for when the file says nothing. THE TEST FOUND A REAL BUG, and it is the interaction I would not have thought to look for. First-match-wins met an empty value: a file with system system /System/Boot/bare.bin matched the first line, handed back nothing, and the machine tried to start a file with no name while a good setting sat underneath. An unusable value is an absent one - which is what "configuration is advice" says, and this is where it earns its keep. cfgBare starts an image with no operating system in it at all, which is what loading an ordinary boot image buys: a program wanting the whole machine is a file like any other, chosen the same way the system is. Three disks differing ONLY in boot.cfg, so each is a test of the file rather than of the machinery under it. |
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Anachronaut
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c312853f8e |
The machine starts itself
stage two CosmOS > saved it read it back, 22 bytes: Stage one hands over to stage two out of a boot slot; stage two mounts the filesystem, finds /System/cosmos.bin, takes the image apart and places its code, its data and its vector table, and jumps to the entry point the vectors named. Nothing placed memory for it. What it loads is an ORDINARY BOOT IMAGE, the same SPBT file the emulator has always been handed. That was the user's call and it is the whole trick: a second stage that loads the machine's normal image format is not a boot-specific mechanism, so bare metal SplitBit stops being a special case. A program wanting no operating system under it is just an image, written under CosmOS like any other, and startable because it is a file. Three things in it worth knowing: - THE ENTRY POINT IS CAUGHT ON ITS WAY PAST. Program Memory cannot be read back, so the boot vector cannot be looked up after being installed; the vector loop notices the one addressed at 0xFC00 and keeps it. - A missing "VEC" is not a fault. An image written before vectors existed simply ends after its data, and then the entry point is zero, which is what every such image has always relied on. - Feature flags that are set mean an image asking for a machine this may not be, and the honest answer to a request that cannot be understood is to refuse rather than to run it anyway. The test records that the system WORKS afterwards rather than that it started. A loaded program running is what says the vector table arrived, because a program reaches the system through SWI and nothing else; the file written and the directory entered say the filesystem and the console came up with it. A second disk has a boot slot and nothing to start, and says so rather than jumping somewhere. |
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Anachronaut
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82adeeb193 |
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. |
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Anachronaut
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d07b23f90b |
Rung 2: the machine starts itself off a disk
Stage one exists and works. It is 330 bytes of program and everything it knows is a thing that will be true forever: which port the disk is on, that a SplitBit disk begins with its own name, and where two numbers sit in that first block. Not what a file is, not what a directory is, not that SBFS has versions. It reads the live boot slot into Program Memory, jumps to the first byte, and prints one character and halts if there is nothing there. It is an ordinary boot image for now, so the whole chain runs on machinery that already exists and the emulator has not been touched. Nothing about it changes when it moves into ROM except who puts it in memory. SplitDisk gained "boot" to write a slot and "bootslot" to choose one, kept apart on purpose: writing a slot and starting from it are different decisions, and joining them would make every write a commitment. A slot is always written WHOLE, because one still holding the tail of what was there before is one whose contents depend on its history, and stage one reads all of it without knowing where the file stopped. Three recorded tests, and the pair is the point: two disks differing only in which slot the superblock names, with payloads that say different things. One prints "booted" and the other does not, so this is a test of CHOOSING a slot rather than a test that some bytes were read. The third boots a disk with no boot area and gets the one character a ROM has room for. Eight more host checks, including that a slot is padded whole. Two things worth recording. The first draft used #Align to put the scratch buffer at 0x8000 and produced a 33K file - thirty two kilobytes of zeroes in something meant to be a ROM. It is an address, not storage, which is exactly what the assembler's own scratch map exists to say. And SplitLint caught the second in code written an hour after the baseline that catches it. In the blit set-up, RSTA writes a source address of zero and then RSTA writes a bank number of zero - two unrelated quantities that are equal by accident, in the most safety critical file in the repository. It is marked with a reason rather than removed. |