Lecture 7 Presented By Dr. Shazzad Hosain Asst. Prof. EECS, NSU Real Mode Memory Addressing • The first 1MB memory is Real memory or the Conventional memory Segment n SS A000h * * * 1MB offset Segment 2 16 bit Segment registers DS 8000h Segment 1 1. 2. 3. 4. 1 MB requires 20 bit address CS 0000h Each segment is 64 KB Offset address is 16 bit or 2 byte Actual address = segment address + offset address Real Mode Memory Addressing • Real mode operation allows to address 1MB of memory space – even for the Pentium microprocessor • This first 1MB memory is called the real memory or the conventional memory • A combination of segment and offset address access the real memory • Segment registers contains the beginning address of any 64KB memory segment • The offset address selects the any location within the 64KB memory space Segment Plus Offset Determines Address To get the real address 1. Pad 0H at the end of segment register 2. Add the offset value CS Offset = 1000H = F000H 10000H F000H 1F000H DS Offset = 1234H = 245FH 12340H 245FH 1479FH 1. Since each segment is 64 K, the offset address can take maximum of FFFFH 2. Once, the beginning address is found in segment registers, ending address is calculated by adding FFFFH with the value of segment register after padding 0H after it. From Intel Microprocessor Default Segment and Offset Registers 1. If CS = 1400H and IP/EIP = 1200 H 2. The microprocessor access instruction from 14000 H+ 1200H = 15200H. Suppose 1. 1000H bytes of code 2. 190H bytes of data 3. 200H bytes of stack Allows relocation Figure 2-4: A memory system showing the placement of four memory segments Figure 2-5 Protected Mode • The segment register now contains a selector • Selector selects a descriptor from a descriptor table • The descriptor describes the memory segment’s location • Two descriptor table – Global Descriptor Table (GDT) – Local Descriptor Table (LDT) Selectors and Descriptors * * * Offset Segment Registers As Selector 1. 2. 3. 4. 16 8 0 8191 * * 2 1 0 31 15 7 GDT/LDT 8192 number of descriptors in each table Each descriptor 8 bytes long, thus table size is 64 KB Selector selects one descriptor Descriptor describes the segment Selectors and Descriptors Offset Segment Register CS/DS/ES 8191 * * 2 1 0 16 8 0 GDT/LDT 0000 0000 0001 0010 0012H 0000 0000 0100 0100 0044H 1111 1111 1111 0000 FFF0H 31 15 7 Selectors and Descriptors 8191 Offset CS/DS/ES 16 8 0 * * 2 1 0 GDT/LDT 31 15 7 Access Right Bits Selectors and Descriptors 8191 Offset CS/DS/ES 16 8 0 * * 2 1 0 31 15 7 GDT/LDT 7 0000 0000 5 1001 1011 0000 0000 00H 6 4 3 A0H C2H 2 1 10H 00H 0 Selectors and Descriptors 8191 Offset CS/DS/ES 16 8 0 * * 2 1 0 31 15 7 Code a descriptor for 80286 that starts at 210000H and ends at 21001FH. This memory segment is a code segment that can be read GDT/LDT 7 0000 0000 0000 0000 21H 5 6 4 3 00H 00H 2 1 00H 1FH 0 Limit = 21001FH – 210000H = 001FH Selectors and Descriptors Code a descriptor for 80286 that starts at 210000H and ends at 21001FH. This memory segment is a code segment that can be read 7 0000 0000 5 1xx1 1x11 0000 0000 21H 6 4 3 00H 00H 2 1 00H 1FH 0 Limit = 21001FH – 210000H = 001FH Selectors and Descriptors • • • • 32 bit / 4 byte base, memory size is 4 GB = 22x210x210x210 AV = 1, means available, 0 means not available D = 1, means 32 bit instructions, 0 means 16 bit instructions G bit or the Granularity bit – G = 0, the limit is from 1 to 1MB in length – G = 1, the limit is multiplied by 4K bytes (appended with 000H). So segment length is 4K to 4G bytes in steps of 4K bytes Selectors and Descriptors 7 01H 5 • G bit or the Granularity bit A 0H 0 D O V 6 00H 4 3 00H 00H 2 1 FFH FFH 0 Start = 01000000H Limit = 0FFFFH End = 0100FFFFH – G = 0, the limit is from 1 to 1MB in length – G = 1, the limit is multiplied by 4K bytes (appended with 000H). So segment length is 4K to 4G bytes in steps of 4K bytes Selectors and Descriptors 7 00H 5 • G bit or the Granularity bit A 0H 1 D O V 6 28H 4 3 00H 00H 2 1 00H 10H 0 Start = 00280000H 00010 000 H Limit = End = 00390000H – G = 0, the limit is from 1 to 1MB in length – G = 1, the limit is multiplied by 4K bytes (appended with 000H). So segment length is 4K to 4G bytes in steps of 4K bytes 7 03H 1G D O 5 A V 0H 00H 4 3 00H 00H 2 1 2FH FFH 0 End = 05FFFFFFH Start = 03000000H • G bit or the Granularity bit 6 Size = 02FFFFFFH Limit = 02FFFH – G = 0, the limit is from 1 to 1MB in length – G = 1, the limit is multiplied by 4K bytes (appended with 000H). So segment length is 4K to 4G bytes in steps of 4K bytes 7 5 03H 1xx1 1G D O 001x A V 0H 6 00H 4 3 00H 00H 2 1 2FH FFH 0 End = 05FFFFFFH Start = 03000000H Size = 02FFFFFFH Limit = 02FFFH References • Chapter 2, Intel Microprocessors – by Brey