Question

Some computer engineering students decided to revise the LC-3 for their senior project. In designing the LC-4, they decided to conserve on device registers by combining the KBSR and the DSR into one status register: the IOSR (the input/output status register). IOSR[15] is the keyboard device Ready bit and IOSR[14] is the display device Ready bit. What are the implications for programs wishing to do I/O? Is this a poor design decision? 8

Answer 1

Some computer engineering students decided to revise the LC-3 for their senior project.  KBSR and the DSR into one status register: the IOSR (the input/output status register). IOSR[15] is the keyboard device Ready bit and IOSR[14] is the display device Ready bit can be done in LC-3.

LC-4 is a poor design.

Explanation:

LC-3, is a type of computer educational programming language, an assembly language, which is a type of low-level programming language.

It features a relatively simple instruction set, but can be used to write moderately complex assembly programs, and is a theoretically viable target for a C compiler. The language is less complex than x86 assembly but has many features similar to those in more complex languages. These features make it useful for beginning instruction, so it is most often used to teach fundamentals of programming and computer architecture to computer science and computer engineering students.

The LC-3 specifies a word size of 16 bits for its registers and uses a 16-bit addressable memory with a 216-location address space. The register file contains eight registers, referred to by number as R0 through R7. All of the registers are general-purpose in that they may be freely used by any of the instructions that can write to the register file, but in some contexts (such as translating from C code to LC-3 assembly) some of the registers are used for special purposes.

When a character is typed:

• Its ASCII code is placed in bits [7:0] of KBDR  (bits [15:8] are always zero)
• The “ready bit” (KBSR[15]) is set to one
• Keyboard is disabled -- any typed characters will be ignored

When KBDR is read:

• KBSR[15] is set to zero
• Keyboard is enabled
• Alternative implementation: buffering keyboard input