1 / 14

A ‘protected-mode’ exploration

A ‘protected-mode’ exploration. A look at the steps needed to build segment-descriptors for displaying a message while in protected-mode. Segment-Descriptor Format. 63. 32. Base[31..24]. G. D. R S V. A V L. Limit [19..16]. P. D P L. S. X. C / D. R / W. A. Base[23..16].

howe
Download Presentation

A ‘protected-mode’ exploration

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. A ‘protected-mode’ exploration A look at the steps needed to build segment-descriptors for displaying a message while in protected-mode

  2. Segment-Descriptor Format 63 32 Base[31..24] G D R S V A V L Limit [19..16] P D P L S X C / D R / W A Base[23..16] Base[15..0] Limit[15..0] 31 0 Legend:DPL = Descriptor Privilege Level (0..3) G = Granularity (0 = byte, 1 = 4KB-page) P = Present (0 = no, 1 = yes) D = Default size (0 = 16-bit, 1 = 32-bit) S = System (0 = yes, 1 = no) X = eXecutable (0 = no, 1 = yes) A = Accessed (0 = no, 1 = yes) code-segments: R = Readable (0 = no, 1 = yes) C = Conforming (0=no, 1=yes) data-segments: W = Writable (0 = no, 1 = yes) D = expands-Down (0=no, 1=yes) RSV = Reserved for future use by Intel AVL = Available for user’s purposes

  3. Example: the ‘vram’ segment • The video display-memory for color text occupies a 32KB physical address-range from 0x000B8000 to 0x000BFFFF • It’s segment-limit can be described with ‘byte’ granularity as equal to 0x07FFF (or with ‘page’ granularity as 0x00007 ) • It needs to be a ‘writable’ data-segment • It’s privilege-level ought to be 0 (restricted)

  4. Descriptor Implementations 00 0 0 92 0B 00 8 0 92 0B 8000 7FFF 8000 0007 Using ‘byte’ granularity Using ‘page’ granularity # vram-segment descriptor using ‘byte’ granularity .word 0x7FFF, 0x8000, 0x920B, 0x0000 # vram-segment descriptor using ‘page’ granularity .word 0x0007, 0x8000, 0x920B, 0x0080

  5. Code and data segments • Our program’s code and data will reside at the base memory-address: 0x00010000 • For simplicity when returning to real-mode, we can keep segment-limits as: 0x0FFFF • Both segments can retain privilege-level 0 • Code-segment: ‘readable’ + ‘executable’ • Data-segment: ‘writable’ + ‘readable’

  6. Descriptors implemented data-segment descriptor code-segment descriptor 00 0 0 92 01 00 0 0 9A 01 0000 FFFF 0000 FFFF Using ‘byte’ granularity Using ‘byte’ granularity # data-segment descriptor using ‘byte’ granularity .word 0xFFFF, 0x0000, 0x9201, 0x0000 # code-segment descriptor using ‘byte’ granularity .word 0xFFFF, 0x0000, 0x9A01, 0x0000

  7. Global Descriptor Table • We can put all of our segment-descriptors into the Global Descriptor Table • Our program executes at privilege-level 0 • Every GDT must have a ‘null’ descriptor • Thus our GDT will need four descriptors .align 8 # the Pentium requires ‘quadword’ alignment theGDT: .word 0x0000, 0x0000, 0x0000, 0x0000 # ‘null’ descriptor .word 0xFFFF, 0x0000, 0x9A01, 0x0000 # code-descriptor .word 0xFFFF, 0x0000, 0x9201, 0x0000 # data-descriptor .word 0x7FFF, 0x8000, 0x920B, 0x0000 # vram-descriptor

  8. GDTR register-format 47 16 15 0 Segment Base-Address Segment Limit 32 bits 16 bits The register-image (48-bits) is prepared in a memory-location… regGDT: .word 0x001F, theGDT, 0x0001 # register-image for GDTR … then the register gets loaded from memory via a special instruction lgdt regGDT # initializes register GDTR

  9. segment-selector format 15 3 2 1 0 INDEX T I RPL 16 bits Legend: RPL = Requested Privilege Level (0..3) TI = Table Indicator (0 = GDT, 1 = LDT) INDEX * 8 = number of bytes in table that precede the descriptor

  10. segment-selectors defined • Assembly language source-code is easier for humans to read if meaningful symbols are used as names for ‘magic’ numbers # These ‘equates’ provide symbolic names for our segment-selectors .equ sel_cs0, 0x0008 # code-segment selector .equ sel_ds0, 0x0010 # data-segment selector ,equ sel_es0, 0x0018 # vram-segment selector

  11. Our ‘pmhello.s’ demo • Use these commands to assemble, link, and install our ‘demo’ program (in class): $ as pmhello.s –o pmhello.o $ ld pmhello.o -T ldscript -o pmhello.b $ dd if=pmhello.b of=/dev/sda4 seek=1 • It also needs a ‘boot-sector’ program that can ‘load’ it at the proper memory-address and then transfer control to its ‘entry-point’

  12. Our ‘quikload.s’ loader • We have provided a ‘boot-sector’ program that you can use in our classroom or labs (it’s not designed to work at other places), or you can use your own ‘loader’ program • Here’s how to assemble, link, and install our ‘quikload.s’ example: $ as quikload.s -o quikload.o $ ld quickload.o -T ldscript -o quikload.b $ dd if=quikloab.b of=/dev/sda4

  13. In-class exercise-set #1 • Find out what will happen if you modify the segment-descriptor for video memory so it uses ‘page’ granularity for its limit-field • Find out what will happen if you do NOT set the ES-register’s segment-limit to 64K before clearing the PE-bit in register CR0 • Find out what will happen if you change the DPL and/or RPL to values other than 0

  14. In-class exercise-set #2 • Redesign the ‘pmhello.s’ program so that it expects to be loaded at a higher address: • Say at address: 0x00040000 (i.e., at 256KB) • Say at address: 0x01000000 (i.e., at 16MB) • You will need to change the ‘disk-address packet’ in our ‘quikload.s’ program so that it will transfer your ‘pmhello.b’ code from the disk to your higher memory address

More Related