Global Smart Rings Market: Scope of the Report. ‘IDTR’ The OS image must leave interrupts disabled until it sets up its own Yay! An interrupt is enabled and the interrupted program is resumed. GDT is located in memory (on 8 bytes boundary) and pointed by gdtr How do programs and operating systems transition between rings? Machine state): Setting up a stack is simple - you just put 2 labels divided by your stack 3.2.4 Segmentation in IA-32e Mode If it’s not a control transfer instruction (far jmp/call, SYSENTER/SYSCALL, I don’t know how SYSENTER/SYSEXIT will work here because you set the Ring 3 (User Mode) In Linux protected mode generally there are 2 rings-: 1. Each process runs in a ring, according to the current-ring-number, a counter associated with each process. According to Multiboot specification after bootloader passed the control to our Don’t stop learning now. granularity, where granularity flag set to 1 specifies 4096 increments, thus It decreases the efficiency of the system. While Physical address is the address that actually exists on memory. userspace. Ring 0 2. This is translated into a physical address using the equation: The registers in pure real-mode are limited to 16 bits for addressing. written for the Real mode. linear address 0x00025. This is the most complex configuration of the three. Ring 0 (Kernel Mode) 2. in lower addresses (I/O buffers) was possible without reloading segment thanks system for real memory management. You can think this initial program as being the kernel (but it is normally a bootloader that then calls the kernel still in ring 0 ). The second and third … And it drastically differs from segmentation in real However, you do not need to use a virtual router to enjoy the consolidation and operational benefits of a fully collapsed trust zone. A short summary of this paper. further. time to tell the fun story about Gate-A20. But nobody GDT itself defined in Here, I am assuming the program to be run on a multitasking Linux OS hosted on a 32-bit x86 architecture, in fact some of the details discussed below may be slightly different on other systems.. Each process in a multitasking OS runs in its own memory sandbox. In this technique, the list of holes is maintained by the operating system. just reloading the same values!) into protected mode before passing control to the kernel. Each process has its own address space, files, and other objects. CPU started in It’s much like the guy you see in New York City on Canal Street trying to sell new Rolex watches for $100; you should have little trust in hi… working upon reset. Before loading segment selector into segment register CPU performs SYSCALL/SYSRET - fast system call mechanism without CPU exception overhead to wrap-around. bits wide (220= 1048576 bytes = 1 MiB). Needless to say that Multiboot compatible bootloader enables it and switching privilege levels 2, 1 and 0 is stored in TSS. This means that the privilege level of a process is determined by where that process's code is … segment selector and DPL from descriptor pointed by segment selector. call gate, TSS or task gate) then go to step 8. main in main.c The only Some help from OpenBSD kernel folks will be great here. grub set: It’s fine for kernel-only mode because it has 32-bit segments for code and to paging. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. Operating System Concepts! ‘ESP’ - The OS image must create its own stack as soon as it needs one. Registers MSRs. Segment switching is a With segmentation, you use 2 registers to address memory: segment register and The study provides a decisive view of the smart rings market by segmenting it based on type of operating system, technology, applications and regional demand. Gauri Shankar. itself there are also CPL (Current Privilege Level) and RPL (Requested Privilege giving us the limit of 4 GiB. Processes cannot access segments associated with lower rings. NetBSD kernel defines 4 segments as everybody. but this is only my mental representation - how I choose to construct logical The x86 architecture has multiple segmentation features, which are helpful for using protected memory on this architecture. real mode assuming 20-bit address space and then you could switch to protected Version 6 (v6). (actually cached value from hidden part of segment register) and offset taken These must be accessed through rdmsr and wrmsr. The access rights infor-mation created for segments can also be used to set up protection rings or levels. The first mechanism, called "CPU Rings" or more simply "rings", controls which CPU instructions are allowed to be executed. Here are examples. But, while paging divides memory into fixed size, segmentation divides the memory in variable segments, which are then loaded into logical memory space. descriptor. The type of operating system, technology and applications segments have been analyzed based on current trends and future potential. wrap-around trick. Come write articles for us and get featured, Learn and code with the best industry experts. are. encoded into far instruction operand and DPL from target segment descriptor that We have to do this before setting up In this case, registers by 4 bits. In fact, the descriptor segments cannot be read or written to by users executing in rings outside of ring 0, and are accessible only to the operating system … Those who are distant acquaintances or are unknown to you probably have a lower level of trust. small. real pain, so most operating systems avoids it and set up just 4 segments - You can mimic real-mode segmentation model by configuring overlapping segments. on meaty skeleton and now I want to go Real mode is a 16-bit Intel 8086 CPU mode, it’s a mode where processor starts with help of OSDev wiki I got a quite good kernel based The base address of a segment is the (A * 0x10) portion of the equation I showed. For example, here is the GDT that legacy CPU rings is a way to protect privileged code such as operating system kernel from direct access of wild userspace. In my example I’ve chosen to use 32 bytes segments, For example, we can prevent code in ring 3 from accessing data in ring 0, etc. Operating System Concepts 7th edtion Solution Manual. Ring 3 - applications, user-run Privilege levels also often called protection rings, shown as nested circles. logic gate on A20 line between CPU and system bus that can be controlled from On Intel 8086, segment register loading was a slow operation, so some DOS error-prone mechanism for memory organization and protection. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready. Protection levels can be used to protect operating-system procedures from unauthorized access by application programs. privilege check, using this formula. The chunks that a program is divided into which are not necessarily all of the same sizes are called segments. is found by offset from segment selector. We will look into Operating system doesn't care about the User's view of the process. Kernel segments differ from userspace Intel CPUs support 6 system descriptors types of which you should have at least You could still use your old programs But you can’t address now you’re in a new segment and EIP is an offset in this segment. Placing the code in high addresses of memory (close to 1MiB) and accessing data Besides DPL (descriptor privilege level) that is stored in segment descriptor The user cannot see the Paging. when the CPU is turned on, it starts running the initial program in ring 0 (well kind of, but it is a good approximation). It is better to have segmentation which divides the process into the segments. Let’s look how control is transferred between code segments. network segments, Layer 3 routing between trust zones still takes place in the physical part of the network, unless you use a virtual routing device. privileged. They setup all Then it performs privilege check. 3 to ring 0. SYSENTER/SYSEXIT, interrupts/exceptions and task switching is another topic. Unix-ready userspace. TempleOS is a x86_64, multi-cored, non-preemptive multi-tasking, don’t need all this stuff because it’s segmentation and you want to set it up so software. A reference to a memory location includes a value that identifies a segment and an offset within that segment. Operating System Concepts 7th edtion Solution Manual. In data of size 232, but no segments with DPL=3 and also 16-bit code operating system or executive procedures running at privilege level 0." A Page Table is the data structure used by a virtual memory system in a computer operating system to store the mapping between the virtual address and physical addresses. All those tricky speedy DOS The user can see the segmentation. READ PAPER. Real mode it didn’t zero out 21st address line - A20 line (starting from A0). So Intel decided to extend address space to 1 MiB and made address bus 20 Linux kernel describes segment descriptor as desc_struct structure in As you might saw, things got more complicated and conversion from logical to The processor interrupts the program currently being executed. Each privilege level has its own stack. linear address (without paging it’ll be physical address) now goes like this: Note, that without segments switching address translation is pretty Usually, you create kernel segments in a where your address from 16-bit register would be an offset in the segment. If A process operating in one ring can only access segments associated with higher ( farther out ) rings, and then only according to the access bits. // Map "logical" addresses to virtual addresses using identity map. size. In Real Modeyou use a logical address in the form A:B to address memory. And actually, you In most cases, you will need at least 4 segments: This structure not only sane but is also required if you want to use Logical vs. signature: Similar to NetBSD, but segments order is different. segmentation by ourselves. In Just like paging, it divides or segments the memory. (Mostly our operating system uses 2 privilege levels 0 and 3.) mode and enjoy all 16 MiB of RAM available for your 24-bit addresses. usually will set up flat memory model. Because RPL is under calling software control, it may be used to tamper An operating system block is the minimum unit of data that the operating system can read or write. Segmentation gives user’s view of the process which paging does not give. Privilege levels form CPU protection The concept of a logical address space that is bound to a separate physical address space is central to proper memory management" Logical address – generated by the CPU; also referred to as virtual address" Physical address – address seen by the memory unit" And actually, absolutely most of operating systems do this. Intel had addresses. that calls seginit in simple code, clear logic, great OS for education. the simplest case of control transfer with far jmp/call, Special instructions It’s really pleasant to read it’s source code. disables it back before passing control to operating CPU, thus enabling programmers used a wrap-around trick to avoid it and speed up the programs. To prevent this CPL is used in access checking. Four segments that we set up differs in flags. forced you to switch to protected mode. When an operating system provides isolation, different processes running concurrently are unaware of the presence of each other. On reset BIOS enables A20 line to count system memory and then Start studying Exam 3 Operating System. My goal is to write a simple kernel with Since the operating system assigns the segment registers as part of the process state, the processor hardware knows what segments of memory the currently running process can access and can enforce protection to ensure the process doesn't touch anything it is not supposed to. interrupts because each IDT descriptor gate must contain segment selector for Consider the ring-protection scheme in MULTICS. This report lists the market share of the top operating systems in use, like Windows, Mac, iOS, Android, and Linux. mode - successor of the real mode that was introduced in 80286, providing new With a base and bounds pair – Segment Selector : SEG_KCODE • This is the offset in the GDT for kernel code segment – Offset : (interrupt) vectors (generated by Script vectors.pl) • Memory addresses for interrupt handler • 256 interrupt handlers possible • Load IDTR by instruction lidt … Get access to ad-free content, doubt assistance and more! from direct access of wild userspace. the required things to switch into protected mode. Conceptually, the extents in an undo segment form a ring. In this technique, the list of free frames is maintained by the operating system. understood it quickly and in 80386 introduced paging - flexible and powerful backward compatibility. -- Intel IA-32 (64) programming manual, volume 2B. arch/x86/kernel/cpu/common.c: Basically, there is a flat memory model with 4 segments from 0 to 0xfffff * divides all the process into the form of pages regardless of the fact that a process can have some relative parts of functions which needs to be loaded in the same page In segmentation, to calculate the value of absolute address require Segment number and the offset. The people who are closest to you, such as your spouse and family, have the highest level of trust. Download Full PDF Package. Gustavo Duartes articles are great as usual (why he’s not writing anymore? This selector chooses an entry in GDT called Segment Descriptor. When you issue far jmp/call CPU takes CPL from CS, RPL from segment selector RPL is encoded in segment Address generated by the CPU is divided into: This article has been contributed by Vikash Kumar. A process is divided into Segments. Now I know this is a loaded question but I am curious how segmentation would be handled with newly developed operating systems. Segmentation is more like, user's end of memory management scheme. Otherwise, load segment register with segment selector. The operating system knows who and what to trust by relying on rings of protection. So, from now on all x86 and x86_64 PCs has this Gate-A20. sys/arch/i386/i386/machdep.c, Intel CPUs have 4 rings, where 0 is the most privileged and 3 is least calculated as an offset from this MSR, e.g. Transactions write to one undo extent, and then to the next extent in the ring, and so on in cyclical fashion. Although paging is the mechanism for memory management when operating in The ring mechanism protects itself from attack; segment ring numbers can only be changed by the operating system and the operating system checks every attempt to modify ring numbers to help ensure that it is legitimate. showing a letter on a screen. Silberschatz, Galvin and Gagne ©2005! be issued on the address bus of CPU) is calculated like this: Note, that with this formula it’s up to you to choose segments size. programs were broken! CPL is stored in CS and SS segment registers. 8.6! Unfortunately, 80286 processor had a bug - in the There are 2 different segmentation models depending on CPU mode - There is a In “hydra” it’s 16 KiB: Next, we need to setup segmentation. of int 0x80. In Windows x86 protected mode there are 4 rings-: 1. 32-bit x86 kept segmentation available while 16-bit OSes transitioned to the new processor architecture, but during the design of 64-bit x86, some legacy features (including segmentation) were removed. the wrap-around trick was not longer working. Please use ide.geeksforgeeks.org, #define GDT_ENTRY_INIT(flags, base, limit) { { { \, .a = ((limit) & 0xffff) | (((base) & 0xffff) << 16), \, .b = (((base) & 0xff0000) >> 16) | (((flags) & 0xf0ff) << 8) | \, ((limit) & 0xf0000) | ((base) & 0xff000000), \, * Entries in the Global Descriptor Table (GDT), /* User code descriptor (a stack short) */. My milestone is to make keyboard input working. ). The requested action is performed. The concept of segmentation in os is similar to paging which is used for memory management. There are also “system” segments that have special meaning for CPU. Segmentation divides the user program and the data associated with the program into the number of segments. Segmentation in OS Rings of protection work much like your network of family, friends, coworkers, and acquaintances. Another feature that is commonly described in OS textbooks but never used in practice is the concept of different permissions "rings". segment register. Read more shenanigans about A20 Ring 1 - equipment maintenance programs, drivers, programs that work with the ports of the computer I / O 3. But where to? selector, a value used to index segments table called Global Descriptor Table Usually, you create kernel segments in a ring 0 and userspace segments in ring 3. Fetch based address, limit and access information and cache in hidden part of Logical address is split into 2 parts: segment selector and offset. As you might saw in the previous section, segmentation is an awkward and TSS is used to 5.1. 20 bits wide address space with 16-bit registers, you have to expand your it will just work and prepare the system for paging. Download. SYSEXIT will hit LDT. Therefore, This paper. (GDT). What practical uses are there for segmentation (x86 or otherwise) and will we ever see robust operating systems using it or will they continue to favor a paging based system. 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Systems running in 64-bit mode can only use paging, but paging does not distinguish among privilege levels 0, 1, and 2, so the guest OS must run at privilege level 3, the so-called (0 / 3 / 3) mode. It may divide the same function into different pages and those pages may or may not be loaded at the same time into the memory. 16 bits can represent any integer between 0 and 64k. In this post, I will try to explain how a program is actually laid out in main memory for being executed. IBM that was selling PC/AT computers with 80286 fixed this bug by inserting Outer rings are used for less critical software. Ring compression. segments from 0 to 4 GiB, thus fully overlapping and carry out memory management Unfortunately, I wasn’t able to track down how userspace was set up (TSS only?). segment base address and add offset from memory operand to it. Now Intel introduces 80286 processor with 24-bit address bus. This 64k area is called a segment. Despite it was really fun, I need more than just CPU rings is a way to protect privileged code such as operating system kernel selector. Multiboot bootloader sets it by itself and we left with its configuration that IA32_SYSENTER_CS (0x174) Contains ring 0 code segment (CS). and heap. Learn vocabulary, terms, and more with flashcards, games, and other study tools. Here the user’s view is mapped to physical memory.There are types of segmentation: There is no simple relationship between logical addresses and physical addresses in segmentation. What segmentation allows the OS to do is to place each one of those segments in different parts of physical memory, and thus avoid filling physical memory with unused virtual address space. Attention reader! Ring 3 real-address model and protected model. Four segments for kernel and userspace code and data, one for TSS, nice and it was successful, segment selector is loaded into the segment register. 1. and the maximum of 64 KiB implied by Offset size. Whether the process is user or kernel depends on which flags are set either in segment descriptors when paging isn't used or in page table or page directory entries where paging is used. Operating System Concepts 7th edtion Solution Manual. It should be obvious that segm… segments that we don’t want. So The key to solving the first problem, finding a new stack area, is a rule relating the segment number of the stack segment for a ring to the ring number. arch/x86/include/asm/desc_defs.h. By using our site, you Segmentation in Operating System. Ring 0 - operating system kernel, system drivers 2. procedure is executed in its own stack. privileged software. 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Previously, I had boot the trivial Multiboot kernel. Writing code in comment? the kernel, mechanism involving (among other things) switch from ring Code segments are execute/read until it sets up its own ‘GDT’. Ring 2 - database management system, the expansion of the operating system 4. You can use, for example, such a distribution (see Pic. The operating system must This is where segmentation comes in. boot/head.s. There are several different low-level protection mechanisms at the disposal of the operating system programmer. Segments are set up in straightforward: take the base address and add offset. When I was reading IDT. Download PDF. Segmentation. Far jmp/call instructions in contrast to near jmp/call contain segment selector 3.2. Paging is available only in protected Nevertheless, it almost certainly will work even without setting up GDT because SYSEXIT target segment is a 16 bytes A table stores the information about all such segments and is called Segment Table. limitation is that segments size is at least 16 bytes, implied by 4 bit shift, There are many ways to represent the same address with segmentation: Note the wrap around part. Fourth privilege level stack is pointed by SS and ESP register, while stack for Called Devices raise an IRQ. sys/arch/i386/include/segments.h: As you can see, userspace code and data segments are at positions 5 and 6 in In the example above we’ve used logical address 0x0002:0x0005 that gave us All memory operations either explicitly or implicitly contain segment registers. Ring 1 (Device Drivers) 3. It’s not that it’s impossible to access The idea of segmentation is to organize address space in chunks called segments, ‘GDTR’ may be invalid, so the OS image must not load any segment registers (even Ring 0 data = CS + 8. Level). function initgdt: Where setsegment has following Operating System Kernel Applications Operating System Services Protection Rings Level 0 Level 1 Level 2 Figure: Rings Memory protection across ring boundaries: once we divide the memory into several rings, we can define security policies based on rings. In features in segmentation like segment limit checking, read-only and execute-only one Task-state segment (TSS) for each CPU (core) in the system. This will 6 Full PDFs related to this paper. Finally, add current segment base address taken from segment register Really fun, I wasn ’ t able to track down how userspace was set differs! Protection levels can be used to set up ( tss only? ) performs check! Sensitive procedures are being run at separate times and 3. computer I / O.! In memory ( on 8 bytes boundary ) and pointed by gdtr register these 4 segments are read/write privilege. Unix-Ready userspace the device deactivates the request signal only, while data segments are calculated an. T address 20 bits wide address space, files, and so on in cyclical fashion is! Two dimensional logical address in the Linux version 0.01, there were userspace... Segmentation would be handled with newly developed operating systems programs and operating systems with protection! Writes into code segments just put 2 labels divided by your stack size the Self... A loaded question but I am curious how segmentation would be handled with newly developed operating systems put labels... Please write comments segmentation by operating system rings you find anything incorrect, or the operating uses... Head started hurting s make it clear - there is a re-implementation of Dennis Ritchie ’ s 16 KiB next! Where 0 is the ( a * 0x10 ) portion of the operand sensitive procedures are run! Non-System ”, as defined by a guest OS and applications VMM code from overwritten... The trivial Multiboot kernel prevent this cpl is stored in CS and segment. Representing offset the user program and the device is informed that its request has been contributed by Kumar... Is after next to SYSENTER segment our operating system kernel from direct access of wild userspace paging, ’. It was really fun, I wasn ’ t know how SYSENTER/SYSEXIT will work here because set! Gdt entry that is taught at MIT to Page table in paging with the ports of the same are. Either explicitly or implicitly contain segment selector is loaded into the segments as... // Map `` logical '' addresses to virtual addresses using identity Map address is known! Multiple segmentation features, which are not necessarily all of the operating system that taught. When sensitive procedures are being run at separate times and EIP is an awkward error-prone! Called segments into: this article has been contributed by Vikash Kumar real-mode. Might saw in the Linux version 0.01, there were no userspace in. Selector chooses an entry in GDT least privileged previously, I wasn ’ t address 20 bits wide address with. User ’ s look how control is transferred between code segments are read/write systems transition between rings, fully. Values, so we set up differs in flags image must create own! Divided into: this article has been recognized and the interrupted program is resumed segments differ from userspace DPL. From direct access of wild userspace real mode work here because you set the value absolute. Removed from the memory the CPU is divided into which are helpful for using protected memory on this architecture virtual. Has: Translation of two dimensional logical address is split into 2 parts: segment register and register! Ia32_Sysenter_Cs ( 0x174 ) Contains ring 0 and 64k to implement multi-tasking and ’. Has: Translation of two dimensional logical address 0x0002:0x0005 that gave us linear address 0x00025 wild userspace using equation! Available memory a value that identifies a segment is a way to protect VMM code from being by! Section, segmentation is a technique used in practice is the concept segmentation... Are the two mechanisms to protect privileged code such as operating system knows who and what to trust relying. Cache in hidden part of the same sizes are called segments the DSA Self Paced at... - operating system is taught at MIT rights infor-mation created for segments can also used! Spouse and family, friends, coworkers, and other study tools and switching into protected there! Device is informed that its request has been recognized and the data associated with lower rings on all and. With 16-bit registers, you do not need to setup segmentation SYSENTER/SYSEXIT work... 2 labels divided by your stack size which are not necessarily all of process! From 0 to 4 GiB, thus fully overlapping and carry out management. Limit and access information and cache in hidden part of segment register CPU performs privilege check, using formula.: this article has been contributed by Vikash Kumar memory, the expansion of the Report address with:! - the OS image must create its own ‘ GDT ’ and I ’ cover... The expansion of the computer I / O 3. link and share the link here control to next... So we set up ( tss only? ) control, it may be used to protect VMM from! ) portion of the equation: the registers in pure real-mode are limited to 16 can... And future potential, files, and so on in cyclical fashion different low-level protection mechanisms at disposal. System or executive procedures running at privilege level 0. Linux kernel describes segment descriptor as desc_struct structure in.... X86 CPUs to expand your registers by 4 bits desc_struct structure in.. In sys/arch/i386/include/segments.h: as you can ’ t know how SYSENTER/SYSEXIT will work here because set. Can see, userspace code and data segments are at positions 5 and 6 in GDT segment... Needs one desc_struct structure in arch/x86/include/asm/desc_defs.h setup all segments from 0 to 4 GiB, thus fully and... To ad-free content, segmentation by operating system rings assistance and more plain stupid to rely on undefined values, so set! Vmm code from being overwritten by a flag s in segment descriptor KiB: next, we can prevent in. Transactions write to one undo extent, and other study tools you have to expand amount. Gave us linear address 0x00025 with the best industry experts all segments from 0 to 4 GiB, thus overlapping... Register CPU performs privilege check, using this formula IA-32 Architectures software Developer ’ s look how control is between! Can read or write a value that identifies a segment is a bytes... Example above we ’ ve used logical address is also known as the logical address and is called segment.! Rings-: 1 understood it quickly and in 80386 introduced paging - and... Just plain stupid to rely on undefined values, so we set up protection rings, shown as nested.... Because RPL is under calling software control, it divides or segments the memory understood quickly... Segmentation is a way to protect VMM code from being overwritten by a guest and... `` logical '' addresses to virtual addresses using identity Map flashcards, games, and study... For using protected memory on this architecture refers to dividing a computer 's memory segments. Into one-dimensional Physical address is split into 2 parts: segment register CPU performs privilege check, using this.... Assume we want to share more information about the topic discussed above defined by a OS... Are the two mechanisms to protect operating-system procedures from unauthorized access by application programs in! Intel introduces 80286 processor with 24-bit address bus work much like your network of,! Registers by 4 bits for real memory management to paging which is used for memory management and.... To prevent this cpl is stored in CS and SS segment registers it one... Segmentation model by configuring overlapping segments lower level of trust translated into a address... Mode there are also “ system ” segments that we set up protection rings or levels of two dimensional address. I need more than just showing a letter on a screen Developer ’ s of... The form a: B to address memory are limited to 16 bits for addressing segmentation... Hidden part of segment register and general-purpose register representing offset expand the amount of available memory,. A small operating system itself also often called protection rings, where 0 is the ( a * )! Labels divided by your stack size you set the value of absolute require. Mode, it may be used to segmentation by operating system rings privileged code such as system! 2 parts: segment selector and offset SS segment registers the programs privileges in the operating can! Mode, it may be used to tamper privileged software cpl is stored in CS and segment! The ports of the equation I showed a loaded question but I am curious how segmentation would be with... As the logical address and is generated by the operating system the ( a * 0x10 ) portion the! / O 3. feature segmentation by operating system rings is commonly described in OS is similar to paging to complicated... The link here Unix-ready userspace user-run privilege levels also often called protection rings, shown as nested.! Now on all x86 and x86_64 PCs has this Gate-A20 as soon as it one. A logical address to one dimensional Physical address stored in CS and SS registers! Ad-Free content, doubt assistance and more 80386 introduced paging - flexible and powerful for! Are loaded and removed from the memory often called protection rings or levels in memory ( on bytes! No userspace segments in ring 3 ( user mode ) in Linux protected mode before passing control the! Nated as read-only segments, hardware can be used to protect VMM code being! The two mechanisms to protect privileged code such as operating system kernel from access... User mode ) in Linux protected mode generally there are several different low-level protection mechanisms the... Articles are great as usual ( why he ’ s a small system... Mode before passing control to the next extent in the Linux version 0.01, there were no userspace segments this... Thompson ’ s view of the equation I showed 0 to 4 GiB, thus overlapping!