Introduction to Embedded Systems
Memory Management:
Memory Management:
Overlays and Virtual Memory
Overlays and Virtual Memory
Lecture 14
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Introduction to Embedded Systems
Memory Management:^ Memory Management:Overlays and Virtual Memory^ Overlays and Virtual Memory
Lecture 14
Introduction to Embedded Systems
How Does a Program Start Running?^ How Does a Program Start Running?
0x3fff 0x
RAM
- CPU’s Program Counter is then
set to the starting address of theprogram and the program beginsexecution.
OS (loader) copies a program frompermanent storage into RAM - on PCsand workstations, the “operating system”copies the program (bits) from disk.
Introduction to Embedded Systems
Solution: Only Load Part of the Program^ Solution: Only Load Part of the Program
One option: programmer breaks code into pieces that fit intoRAM
Pieces, called
overlays
, are loaded and unloaded by the program
Does not require OS help
RAM
0xff 0x
0x01ff 0x01000x00ff 0x
load one overlay and rununtil other overlayis needed
Program
Introduction to Embedded Systems
ARM Linker Supports Overlays^ ARM Linker Supports Overlays •
Overlay manager
loads an overlay segment when it is not
in RAM
Supports Static and Dynamic Overlays
Static overlays
One root segment
2 or more memory partitions (1 for the root partition which is alwaysin RAM)
Within each partition, any number of overlay segments
Only 1 overlay segment can be in a partition at a given time
Application writer specifies what is in each partition and segment^ root segment
2_
2_
2_
2_
1_
1_
1_
1_
high address low address
- segments 1_1 .. 1_4 share the same
memory area
- so do segments 2_1 .. 2_
Introduction to Embedded Systems
Dynamic Overlay^ Dynamic Overlay •
Include
re-location information
with each overlay segment
Have overlay manager allocate memory for an overlaysegment when it is first loaded
Load and unload overlay segments by explicit calls tooverlay manager
Each overlay segment is given its own name - linker linkseach as if it were in its own partition
Introduction to Embedded Systems
Problems with Overlays^ Problems with Overlays •
Difficult for programmer to manage
Not general - management of resources (RAM) is anoperating system issue
But many embedded systems do not have the space or power for a“real” OS
Introduction to Embedded Systems
The Memory Pyramid^ The Memory Pyramid
Introduction to Embedded Systems
How Does VM Work?^ How Does VM Work? •
Two memory “spaces”
Virtual memory space
what the program “sees”
Physical memory space
- what the program runs in (size of RAM)
On program startup
OS copies program into RAM
If there is not enough RAM, OS stops copying program and starts itrunning with only a portion of the program loaded in RAM
When the program touches a part of the program not in physicalmemory (RAM), OS catches the memory abort (called a
page fault
)
and copies that part of the program from disk into RAM
In order to copy some of the program from disk to RAM, OS mustevict parts of the program already in RAM
- OS copies the evicted parts of the program back to disk
Introduction to Embedded Systems
Example (^ Example (
con'tdcon'td
Need VANeed VA
toto
PA mappingsPA mappings
bne
0x
add
r10,r1,r
sub
r3,r4,r
sw
r5,0x0c
0x000x040x080x0C0x100x140x180x1C
0x000x040x080x0C
add
r1,r2,r
sub
r2,r3,r
lw
r2,
0x
mult
r3,r4,r
add
r1,r2,r
sub
r2,r3,r
lw
r2,
0x
mult
r3,r4,r
Virtual Address Space
Physical Address Space
0x
0x
0x
0x
0x
0x
0x
Disk
0x
Disk
0x
Disk
Disk
VA->PA
Introduction to Embedded Systems
Example (^ Example (
con'tdcon'td
AfterAfter
handling ahandling a
page faultpage fault
bne
0x
add
r10,r1,r
sub
r3,r4,r
sw
r5,0x0c
0x000x040x080x0C0x100x140x180x1C
0x000x040x080x0C
add
r1,r2,r
sub
r2,r3,r
lw
r2,
0x
mult
r3,r4,r
bne
0x
sub
r2,r3,r
lw
r2,
0x
mult
r3,r4,r
Virtual Address Space
Physical Address Space
0x
Disk
0x
0x
0x
0x
0x
0x
0x
Disk
0x
Disk
Disk
VA->PA
Introduction to Embedded Systems
Basic VM Algorithm^ Basic VM Algorithm •
Program asks for virtual address
Computer translates
virtual address
(VA) to
physical
address
(PA)
Computer reads PA from RAM, returning it to program
0x000x040x080x0C
add
r1,r2,r
sub
r2,r3,r
lw
r2,
0x
mult
r3,r4,r
RAM
0x
0x
0x
0x
0x
0x
0x
Disk
0x
Disk
0x
Disk
Disk
VA->PA
Processor
(running program)
Instructions
(or data)
Virtual address
Introduction to Embedded Systems
Page Tables^ Page Tables
Table which holds VA -> PA translations is called the
page table
In our current scheme, each word is translated from a virtualaddress to a physical address
How big is the page table?
0x000x040x080x0C
add
r1,r2,r
sub
r2,r3,r
lw
r2,
0x
mult
r3,r4,r
RAM
0x
0x
0x
0x
0x
0x
0x
Disk
0x
Disk
0x
Disk
Disk
VA->PA
Processor
(running program)
Instructions
(or data)
Virtual address
Introduction to Embedded Systems
Virtual Pages & Physical Page Frames^ Virtual Pages & Physical Page Frames
0x7fff0x6fff0x5fff0x4fff0x3fff0x2fff0x1fff0x0fff0x
0x3fff0x2fff0x1fff0x0fff0x
Virtual Address Space
Physical Address Space
virtual page 0x000-0xfff
4KB
physical page frame
Introduction to Embedded Systems
Page Frames^ Page Frames •
Every address within a virtual page maps to the samelocation within a physical page frame
In other words, bottom
log 2
(page size in bytes) is
not
translated