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rep lodsb
Personal blog · rep-lodsb.mataroa.blog/bloghardwaresystems
rep lodsb is a personal blog at rep-lodsb.mataroa.blog about hardware and systems. Since 2022, 4 different Hacker News readers have linked it 12 times, most often “The 286's internal registers (2022)”. The last post was in 2022, but the archive is still widely read.
Most-linked posts?
- The 286's internal registers (2022)4 people · 5 mentions · first linked 2022-08-13
- Intel 286 secrets: ICE mode and F1 0F 042 people · 7 mentions · first linked 2022-08-12
What Hacker News says?
It had ICE mode (precursor to SMM, but used for production testing and low-level debugging), and according to this article, the pins were exposed at least on some of the chips you could buy: https://www.rcollins.org/ddj/Jan97/Jan97.html >On the standard Intel 80386 DX, asserting the undocumented pin at location B6 will cause the microprocessor to halt emulation and enter ICE mode. [this is written from an ICE perspe…
Comment linking /blog/intel-286-secrets-ice-mode-and-f1-0f-04/
on “A CT scanner reveals surprises inside the 386 processor's ceramic package”Submitted Intel 286 secrets: ICE mode and F1 0F 04 as a story · 121 points
HN discussionYes, and it's not quite the same as a normal "INT 01h". It causes a debug exception, which may enter ICE mode if it is enabled (undocumented bit in DR7, or PMCR on Pentium), otherwise it invokes interrupt 1, but without checking the privilege level on the IDT entry, or the interrupt redirection bitmap in V86 mode. https://www.rcollins.org/secrets/opcodes/ICEBP.html IIRC, older versions of the Linux kernel had a secu…
Comment linking /blog/intel-286-secrets-ice-mode-and-f1-0f-04/
on “Why is the x86 undefined instruction called ud2? Why 2?”Submitted The 286's internal registers (2022) as a story · 73 points
HN discussion286 doesn't support paging, only segmentation. I have read the manuals extensively, and also discovered what may be the last undocumented feature, almost 40 years after the chip being released. [ https://rep-lodsb.mataroa.blog/blog/intel-286-secrets-ice-mo... ] Not saying this to brag, just to establish that I probably know more about that chip than the average poster. And you're also posting plainly wrong claims el…
Comment linking /blog/intel-286-secrets-ice-mode-and-f1-0f-04/
on “80286 ATX mainboard based on the IBM 5170 AT PC”Submitted The 286's Internal Registers as a story · 5 points
HN discussionjust posted another article that goes into that :) https://rep-lodsb.mataroa.blog/blog/the-286s-internal-regist...
Comment linking /blog/the-286s-internal-registers/
on “Intel 286 secrets: ICE mode and F1 0F 04”Submitted Intel 286 secrets: ICE mode and F1 0F 04 as a story · 2 points
HN discussionNo, because it depends on speculative execution and performance counters. The earliest microarchitecture where this might be possible would be the P6? And I don't think there are any more x86-16 opcodes left to be discovered. Some time ago I did quite a bit of experimentation with the 80286 to find what is probably the last one[1], and also looked closely at its "entry point PLA" in die shots, which maps opcodes to…
Comment linking /blog/intel-286-secrets-ice-mode-and-f1-0f-04/
on “Speculating the entire x86-64 instruction set in seconds (2021)”Submitted The 286's Internal Registers as a story · 1 points
HN discussionThe 80186 already added dedicated logic for address computation, as well as multiply/divide and repeated shift/rotate (no barrel shifter, but only one cycle for each shifted bit instead of several as on the 8086). It also had the extended instructions, except for those related to protected mode. However the microcode format remained essentially the same[1], so I don't think there was a fundamental redesign in either…
Comment linking /blog/the-286s-internal-registers/
on “Reverse-engineering the conditional jump circuitry in the 8086 processor”186 didn't have protected mode :) On the 286, the only way out of protected mode was via a CPU reset. The keyboard controller (which was a separate chip on the motherboard, an 8042 microcontroller) had a documented command to do that, and one of the first things the BIOS entry point did was to check a flag indicating that the reset was intended to exit from protected mode, if so it gave control back to the applicati…
Comment linking /blog/intel-286-secrets-ice-mode-and-f1-0f-04/
on “32-Bit DOS Applications”