It is good programming practice to use the operating system calls provided, rather than trying to manipulate the hardware directly. This avoids compatibility problems with future releases , and leaving the operating system in an unstable state. We are therefore NOT going to encourage encouraging the reader to 'poke to the screen' or 'disable the operating system! However we will ' We will encourage you to use these good programming practise and attempt to give as full a description of the hardware that exists as we understand it.
...
| Note | ||||
|---|---|---|---|---|
| ||||
The hardware registers have only one function. They can either be written to, or read from, (but not both). With this constraint it would be have been possible to lose track of register contents. To get around this potential problem, the operating system use addresses duplicates these values by using soft copies of that can always be read, to check out the contents of the hardware registers. Addresses $0400 to $04FF are used to store the 'soft copies' of the values in the BLINK registers, with the low byte of the address being taken from the address of the I/O port. These soft copies can always be read, to check out the contents of the hardware registers. |
| Warning | ||
|---|---|---|
| ||
Always write to the soft copy of a register before writing to the hardware register itself to ensure that the duplicate entry is valid. Reasons are explained below. |
...