Project 4Kristoffer Robin Stokke

Direct Memory Access

• New hardware device to toy with! • Hardware controller with direct access to

physical devices as well as RAM• Used to transfer data from floppy to (physical)

memory in our project!• See the code in floppy.c

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Direct Memory Access

• Limitations – Transfers to memory must be within the first 16

MBs!– Transfers must not cross a 64kB boundary!– All addresses are physical!– See the P4 FAQ!

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Processes in P4

• User mode• Virtual address space

– ?

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Virtual Address Space

• Each process «believes» to run in a separate address space entirely dedicated to itself

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Shell Virtual RAM

0x0

0xFFFFFFFF

Virtual Address Space• Behind the scenes, the MMU maps virtual addresses

to physical addresses using paging mechanisms.

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Shell Virtual RAM

0x0

0xFFFFFFFF

Physical RAM

0x0

0xFFFFFFFF

Virtual Address Space

• Code to set up processes with paging enabled have been given see kernel.c and paging.c

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Kernel

Process

Kernel

Process data, code, stack

Kernel

Processes ThreadsPhysical memory

0x1000000

1 – to - 1 1 – to - 1

0x0 0x0 0x0

Process Management

• Dynamic loading – how?– Simple filesystem on floppy

containing process directory– Implement readdir()– Implement loadproc()– Who calls these functions?

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readdir(uint32_t* buf)

• Systemcall called from shell• Copies data from floppy to memory (buf)

using DMA– From where?

• Is there a problem with that?– Explain in design!

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loadproc()• Systemcall by shell to load a

process• Allocate space for data (see

memory.c)– Enough for code / data + 8kB stack!

• Load data into memory from floppy

• Use create_process() to setup the process18. april 2023 10

Keyboard

• Keyboard is connected to the CPU like the interrupt controller– IRQ1 signals keypress– Key can be read from port 0x60

• Code set up for you in entry.S – Calls keyboard_interrupt(), which calls appropriate

handler, which might call putchar()

• Design getchar(), putchar() and init()18. april 2023 11

Remember..

• Lots of small things that can go wrong!– Attempt to identify such sections and design

correctly to avoid them

• Use the makefile to test the different implementations as stated in the project description

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Snake x86

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A Word of Warning

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• Only one delivery; goes for printed documents as well as deliveries by mail.

• Correctors now take the first of the delivered items to avoid duplicate work.

• Is this OK?

Good luck!

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