Input and Output

Catalogue

1. 1. Input and Output (Unit 4.5)
   1. 1.1. Screen Memory Map
      1. 1.1.1. Screen
      2. 1.1.2. How to set a pixel
   2. 1.2. Keyboard Memory Map

Input and Output (Unit 4.5)

We will learn to use bits to manipulate the peripheral devices.

Screen Memory Map

There is a display unit (256 * 512, b/w)

The Screen Memory Map is a sequence of 16-bit values, each of which is called “a word”. Altogether we have 8k (8192), 16-bit words, because there should be this much bits (one for each pixel).

Note that the read/write operations are only 16-bit, not individual bits.

Screen

We are going to implement the memory map with a chip called Screen. It’s an 8k chip, behaving exactly as a memory unit.

When we build the overall computer, this chip will be part of the data memory (or RAM). In other words, Screen is only one chip among a bunch of other chips.

To access the Screen chip in RAM, the addressing needs to add the base address (say, 16384).

How to set a pixel

To set a pixel (row, col) on or off:

1. word = Screen[32 * row + col / 16]
word = RAM[16384 + 32 * row + col / 16]
2. Set the col % 16 -th bit of word to zero or one.
3. Commit word to RAM

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Keyboard Memory Map

This is the chip which a physical keyboard is associated with. Its memory map needs no more than 16 bits, which is a single register. Every key has an agreed-upon value as a scan code.

When I lift my finger or the keyboard is idle, the memory map returns zero.

In Hack computer, it is in RAM[24576].