Notes on BIOS usage ------------------- - DOS (including 6.22/7.1) does not need INT 15h or INT 1Ah. Most other operating systems require INT 15h to detect installed memory. - OS/2 (WSeB/MCP/ACP) and Windows 98 SE are some of the very few operating systems which use the El Torito floppy emulation. - NetWare 5.1 is one of the *extremely* few users of El Torito hard disk emulation. - Keystroke check (INT 16h, fn 01h/10h) always enables interrupts on return. DOS POWER.EXE depends on that in some situations. - MS-DOS 6.2/V is a rare user of the INT 15h keyboard intercept routines. - Some software uses the model byte at F000:FFFE to determine the system type (PC-DOS 3.0, Norton Utilities 8). Other software first tries INT 15h, fn C0h instead (PC-DOS 3.1, MSD). - DOS 4.01 (both IBM and Microsoft) calls INT 13h to read from disk with less than 100 bytes of stack space early in the boot sequence. This tends to be a problem especially for the SATA and SCSI code paths. - Very few guests use the 32-bit PCI BIOS interface. One is OS/2 (but falls back), another is Etherboot. - OS/2 is the only known guest which can run the 16-bit PCI BIOS in protected mode (but only if the 32-bit PCI BIOS is unavailable). - NetWare 6.x is the only known guest which uses the PCI BIOS service to read the IRQ routing table. - Any disk reads which use bus-master DMA (AHCI, IDE BM) must use VDS (Virtual DMA Services) when present. Otherwise any reads/writes when the real mode addresses don't map directly to physical addresses will fail horribly. DOS 6.x with EMM386 is a good testcase (esp. loading drivers into UMBs). - Many older OSes (especially UNIX based) require the FDPT to contain physical ATA disk geometry; for that reason, disks smaller than ~500MB are easiest to use. Otherwise a "large" BIOS disk option would be required. - Some really old OSes (Xenix circa 1986-7) do not understand the EBDA idea and clear the memory. For those, the FDPT must be in the BIOS ROM area, or the OS will destroy it (even when it's at 0:300 in the IVT). - Windows NT (including XP) uses INT 13h/08h to obtain the DPT for each floppy drive. NT assumes a 13-byte DPT which includes the number of tracks. NT will refuse to read more tracks than the DPT specifies and formats as many tracks as the DPT specifies. - Windows 98 SE boot CD uses 32-bit registers in real mode and will fail in mysterious ways if BIOS trashes high bits of EAX (and likely others). - PC DOS 6.x/7.x QCONFIG is a rare user of INT 16h fn 0Ah (read keyboard ID). - DOS POWER.EXE uses the real mode APM interface, OS/2 APM.SYS uses the 16-bit protected mode APM interface, and Windows 9x uses the 32-bit protected mode APM interface. - Windows 98 is one of the few APM 1.2 users; Windows 95 uses APM 1.1, while newer systems prefer ACPI. - QNX4 calls 16-bit protected-mode PCI BIOS in an environment where ESP is 16-bit but SS is a 32-bit stack segment. In such environments, using the ENTER/LEAVE sequence is fatal if the high word of EBP is non-zero (which it will be with QNX 4.25). LEAVE propagates the high word of EBP into ESP with fatal consequences. - Plan 9 also runs 16-bit code with a 32-bit stack segment, except Plan 9 thinks it counts as real mode. Same ENTER/LEAVE problem as above. - AIX 1.3 is a rare user of INT 15h/89h (switch to protected mode) service. - IBM OS/2 1.0/1.1 (but not other versions!) attempt to execute a 286 LOADALL instruction. LOADALL must be emulated for OS/2 to work properly. HIMEM.SYS version 2.03 and later also contains 286 LOADALL code but this will not be executed on 386+ processors. - IBM and Microsoft OS/2 1.0 use CMOS shutdown status 9 to get back from protected mode without having called INT 15h/87h at all. That makes the status 9 handling a public interface (just like codes 5 and 0Ah) which has to be compatible with other BIOS implementations. - Windows NT 3.5 and 3.51 with MPS HAL requires that INT 15h/E820h return the I/O APIC range as reserved, or not return any ranges at all just below 4GB. Otherwise the NT kernel will crash early during init due to confusion about the top of memory. 286 BIOS -------- For testing purposes, it's quite useful to have a BIOS that can run in a classic PC/AT environment with a 286 CPU. This forces various changes, not always obvious: - C code can be easily compiled to produce 286-compatible object code - 32-bit BIOS services such as APM or PCI BIOS are irrelevant - PCI cannot be supported because it requires 32-bit port I/O - AHCI cannot be supported because it requires 32-bit port I/O and PCI - Switching to protected mode must be done using LMSW instead of CR0 - Switching back to real mode must reset the CPU (currently triple fault) and regain control by setting up the CMOS shutdown status byte Notes on BIOS implementation ---------------------------- - To return values from functions not declared as __interrupt, the arguments may need to be declared volatile (not ideal, but does the job). - The way the POST code selectively clears or doesn't clear memory is extremely suspect and will need reworking. - Need to review string routines wrt direction flag (should be OK now). - Need to review CMOS access wrt interrupts (possible index reg change by an interrupt handler). - The POST code zeroes the entire BDA, and then various bits zero specific parts of the BDA again. That's a waste of time. - After a reset, all interrupts are unmasked. Not sure if that's OK. - BCC mishandles the following (where buf is an uint8_t array): lba=buf[0x2B]*0x1000000+buf[0x2A]*0x10000+buf[0x29]*0x100+buf[0x28]; The buf[x]*100 expression should end up being of type signed int, which causes the sign to be incorrectly propagated. BCC incorrectly keeps the type unsigned. - The PCI BIOS services are implemented in C, compiled twice as 16-bit and 32-bit code. This reduces the development effort and significantly lowers the risk of discrepancies between 16-bit and 32-bit implementation. Care must be taken because the 16-bit implementation can be executed in both real and protected mode. - APM can be in theory implemented only once for real, 16-bit protected and 32-bit protected mode. Unfortunately this is very inconvenient in C since the default stack size changes between 16-bit and 32-bit callers. Therefore real mode APM (which supports most functions) is implemented in C and protected-mode APM is written in assembler for both 16-bit and 32-bit calls, with a small 32->16 thunk. - The -of switch can be used to avoid generating ENTER/LEAVE instructions. This appears to be an undocumented and perhaps unintentional side effect. Code size notes (code as of 7/6/2011): The following values are the size of the _TEXT segment, i.e. only C code; data defined in C is not included, neither are assembly modules. Options: Size (hex): -------- ----------- -0 -zu -s -oas -ecc 631A -3 -zu -s -oas -ecc 5C1E -0 -zu -s -oas 578A -3 -zu -s -oas 5452 Both generating 386 code and using register-based calling convention for internal functions brings significant size savings (15% when combined).