1 |
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2 | Notes on BIOS usage
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3 | -------------------
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4 |
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5 | - DOS (including 6.22/7.1) does not need INT 15h or INT 1Ah. Most other
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6 | operating systems require INT 15h to detect installed memory.
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7 |
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8 | - OS/2 (WSeB/MCP/ACP) and Windows 98 SE are some of the very few operating
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9 | systems which use the El Torito floppy emulation.
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10 |
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11 | - NetWare 5.1 is one of the *extremely* few users of El Torito hard disk
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12 | emulation.
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13 |
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14 | - Keystroke check (INT 16h, fn 01h/10h) always enables interrupts on return.
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15 | DOS POWER.EXE depends on that in some situations.
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16 |
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17 | - MS-DOS 6.2/V is a rare user of the INT 15h keyboard intercept routines.
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18 |
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19 | - Some software uses the model byte at F000:FFFE to determine the system
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20 | type (PC-DOS 3.0, Norton Utilities 8). Other software first tries INT 15h,
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21 | fn C0h instead (PC-DOS 3.1, MSD).
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22 |
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23 | - DOS 4.01 (both IBM and Microsoft) calls INT 13h to read from disk with less
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24 | than 100 bytes of stack space early in the boot sequence.
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25 |
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26 | - Very few guests use the 32-bit PCI BIOS interface. One is OS/2 (but falls
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27 | back), another is Etherboot.
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28 |
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29 | - OS/2 is the only known guest which can run the 16-bit PCI BIOS in protected
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30 | mode (but only if the 32-bit PCI BIOS is unavailable).
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31 |
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32 | - NetWare 6.x is the only known guest which uses the PCI BIOS service to read
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33 | the IRQ routing table.
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34 |
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35 | - Any disk reads which use bus-master DMA (AHCI, IDE BM) must use VDS
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36 | (Virtual DMA Services) when present. Otherwise any reads/writes when the
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37 | real mode addresses don't map directly to physical addresses will fail
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38 | horribly. DOS 6.x with EMM386 is a good testcase (esp. loading drivers
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39 | into UMBs).
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40 |
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41 | - Many older OSes (especially UNIX based) require the FDPT to contain
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42 | physical ATA disk geometry; for that reason, disks smaller than ~500MB are
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43 | easiest to use. Otherwise a "large" BIOS disk option would be required.
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44 |
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45 | - Some really old OSes (Xenix circa 1986-7) do not understand the EBDA idea
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46 | and clear the memory. For those, the FDPT must be in the BIOS ROM area, or
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47 | the OS will destroy it (even when it's at 0:300 in the IVT).
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48 |
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49 | - Windows NT (including XP) uses INT 13h/08h to obtain the DPT for each floppy
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50 | drive. NT assumes a 13-byte DPT which includes the number of tracks. NT will
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51 | refuse to read more tracks than the DPT specifies and formats as many tracks
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52 | as the DPT specifies.
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53 |
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54 | - Windows 98 SE boot CD uses 32-bit registers in real mode and will fail in
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55 | mysterious ways if BIOS trashes high bits of EAX (and likely others).
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56 |
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57 | - PC DOS 6.x/7.x QCONFIG is a rare user of INT 16h fn 0Ah (read keyboard ID).
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58 |
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59 | - DOS POWER.EXE uses the real mode APM interface, OS/2 APM.SYS uses the 16-bit
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60 | protected mode APM interface, and Windows 9x uses the 32-bit protected mode
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61 | APM interface.
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62 |
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63 | - Windows 98 is one of the few APM 1.2 users; Windows 95 uses APM 1.1, while
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64 | newer systems prefer ACPI.
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65 |
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66 | - QNX4 calls 16-bit protected-mode PCI BIOS in an environment where ESP is
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67 | 16-bit but SS is a 32-bit stack segment. In such environments, using the
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68 | ENTER/LEAVE sequence is fatal if the high word of EBP is non-zero (which
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69 | it will be with QNX 4.25). LEAVE propagates the high word of EBP into ESP
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70 | with fatal consequences.
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71 |
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72 | - Plan 9 also runs 16-bit code with a 32-bit stack segment, except Plan 9
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73 | thinks it counts as real mode. Same ENTER/LEAVE problem as above.
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74 |
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75 |
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76 | Notes on BIOS implementation
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77 | ----------------------------
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78 |
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79 | - To return values from functions not declared as __interrupt, the arguments
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80 | may need to be declared volatile (not ideal, but does the job).
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81 |
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82 | - The way the POST code selectively clears or doesn't clear memory
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83 | is extremely suspect and will need reworking.
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84 |
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85 | - Need to review string routines wrt direction flag (should be OK now).
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86 |
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87 | - Need to review CMOS access wrt interrupts (possible index reg change by
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88 | an interrupt handler).
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89 |
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90 | - The POST code zeroes the entire BDA, and then various bits zero specific
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91 | parts of the BDA again. That's a waste of time.
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92 |
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93 | - After a reset, all interrupts are unmasked. Not sure if that's OK.
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94 |
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95 | - BCC mishandles the following (where buf is an uint8_t array):
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96 | lba=buf[0x2B]*0x1000000+buf[0x2A]*0x10000+buf[0x29]*0x100+buf[0x28];
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97 | The buf[x]*100 expression should end up being of type signed int, which
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98 | causes the sign to be incorrectly propagated. BCC incorrectly keeps
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99 | the type unsigned.
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100 |
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101 | - The PCI BIOS services are implemented in C, compiled twice as 16-bit and
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102 | 32-bit code. This reduces the development effort and significantly lowers
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103 | the risk of discrepancies between 16-bit and 32-bit implementation. Care
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104 | must be taken because the 16-bit implementation can be executed in both
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105 | real and protected mode.
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106 |
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107 | - APM can be in theory implemented only once for real, 16-bit protected and
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108 | 32-bit protected mode. Unfortunately this is very inconvenient in C since
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109 | the default stack size changes between 16-bit and 32-bit callers. Therefore
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110 | real mode APM (which supports most functions) is implemented in C and
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111 | protected-mode APM is written in assembler for both 16-bit and 32-bit calls,
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112 | with a small 32->16 thunk.
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113 |
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114 | - The -of switch can be used to avoid generating ENTER/LEAVE instructions.
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115 | This appears to be an undocumented and perhaps unintentional side effect.
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116 |
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117 |
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118 | Code size notes (code as of 7/6/2011):
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119 |
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120 | The following values are the size of the _TEXT segment, i.e. only C code;
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121 | data defined in C is not included, neither are assembly modules.
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122 |
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123 | Options: Size (hex):
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124 | -------- -----------
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125 | -0 -zu -s -oas -ecc 631A
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126 | -3 -zu -s -oas -ecc 5C1E
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127 | -0 -zu -s -oas 578A
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128 | -3 -zu -s -oas 5452
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129 |
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130 | Both generating 386 code and using register-based calling convention for
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131 | internal functions brings significant size savings (15% when combined).
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