Week 3

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1. Linux

Implement a simulated shell, called nsh. When running the operating system, write nsh in command line to start. In this simulated shell, we can also run some commands as in the original shell.

 Implement the buddy allocation (kernel/buddy.c), which allocates and free file structs so that xv6 can have more open file descriptors.

  • What is buddy allocation?
  • In a buddy system, the entire memory space available for allocation is initially treated as a single block whose size is a power of 2. When the first request is made, if its size is greater than half of the initial block then the entire block is allocated. Otherwise, the block is split in two equal companion buddies. If the size of the request is greater than half of one of the buddies, then allocate one to it. Otherwise, one of the buddies is split in half again. This method continues until the smallest block greater than or equal to the size of the request is found and allocated to it.
  • In this method, when a process terminates the buddy block that was allocated to it is freed. Whenever possible, an unallocated buddy is merged with a companion buddy in order to form a larger free block. Two blocks are said to be companion buddies if they resulted from the split of the same direct parent block.

Design how to create address space. The central data structure is pagetable_t (it can be used for both kernel page table and process page table), which is really a pointer to a RISC-V root page-table page. The walk function will find the PTE for a virtual address, and mappages, which installs PTEs for new mappings. 


2. Bootloader

Search for the knowledge about u-boot on the Internet, watch videos about u-boot uploaded by others online, and learn how to install and write u-boot based on the tutorial.

3. SoC

  1. Construct Control Unit, and test ALU control and register file (commits on Feb 10, 2020)
  2. Test the ALU with new extension support and fix the bugs (commits on Feb 9, 2020)
  3. Confirm the function of SDRAM to DDR Component provided by Digilent on board (commits on Feb 12, 2020)
    1. Advantages: It can be directly used to save time
    2. Disadvantages: minimum writing time is 260ns and reading time is 210ns, which causes huge memory operation latency. In addition, it doesn’t have the output for a normal bus that guarantees the communication between memory and CPU core
    3. Problem existing: After reading or writing many times to memory, it will obviously lose data (maybe caused by refresh rate of DDR2 SDRAM in this FPGA)
  4. Construct register between each stage used in pipeline processor
  5. Test Control Unit and fix the bugs (commits on Feb 15, 2020)

Figure 1. Test of ALU




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Week 1

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1. Linux Install the running environment (including riscv32-unknown-elf-gcc and qemu-system-riscv32). riscv32-unknown-elf-gcc is the cross compile, which can compile the operating system source code so that it can be used in 32 bit RISC-V. Qemu-system-riscv32 can simulate the 32 bit RISC-V environment on Ubuntu. Therefore, we can test the correctness of operating system without a completed CPU.  After finishing the installation, write a simple C program to test the correctness. Firstly, use the riscv32-unknown-elf-gcc to compile the source code and generate an executable file. Then, use qemu-system-riscv32 to run this executable file. This process is shown in following figure. 2. Bootloader Read the book called <Computer Organization and Design RISC-V Edition: The Hardware Software Interface>. This is a thick book for me to read it because I lack knowledge of computer systems. I particularly focused on the second part <Instruction: Language...
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Week 2

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1. Linux Implement several shell commands, such as sleep , pingpong , primes , find and xargs . 1. sleep can pause the operating system for a given time. (eg. sleep 100: the system will pause for 10 second) 2. pingpong can transmit a byte message between two processors via a pair of pipes, parent_pipe and child_pipe , one for each direction (in and out). The parent process will write message to parent_pipe[1] and the child receive this message from parent_pipe[0] . Then child responds with writing a message to child_pipe[1] and parent can read from child_pipe[0]. 3. primes is based on the idea Unix Pipes .  The first process feeds the numbers 2 through 35 into the pipeline. For each prime number, you will arrange to create one process that reads from its left neighbor over a pipe and writes to its right neighbor over another pipe. All the numbers can be divided by the prime number will be discarded as shown in the following figure.  Figure 1: the function of...
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Week 4

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1. Linux Implement lazy allocation, which will only allocate resource until it is actually needed. That is, sbrk function doesn't allocate physical memory, but just remembers which addresses are allocated. When the process first tries to use any given page of memory, the CPU generates a page fault, which the kernel handles by allocating physical memory, zeroing it, and mapping it. 2. Bootloader It was found that the u-boot transplantation tutorial on the Internet was performed on the Samsung template, which is different from the source code used by risc-v. 3. SoC Discuss how the operating system communicate with the CPU (system call) Search information to solve the problem of DDR2 SDRAM, which cause the whole project not to meet the expectation progress. Replace on chip DDR2 RAM with LUT RAM and Block RAM Can not support the compile file of operating system. Therefore, memory support is necessary
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