Chapter 31 ½ A Dual-Processor Desk-Top Computer: The HP 9845A 531

The control bytes and their associated conventions amount to a command set for the alphanumeric display. Their employment allows efficient use of the memory allocated to the CRT display buffer. Rather than structuring the buffer to be a character-for-character image of the display, the buffer contains a compacted version of the data. For instance, the blanks to the right of a line are supplied automatically by the display itself, following an end-of-line (EOL) control character. Other control bytes instruct the alphanumeric interface where in the buffer to begin the display; control the location of the cursor; and specify underlining or blinking.

The size chosen for the display buffer is large enough to contain enough characters to fill an entire display. But because of efficient allocation of memory (e.g., suppression of trailing blanks by EOL control characters) the buffer is rarely full and can be loaded with more lines of information than the CRT can display at one time. The display buffer can hold four pages of average BASIC statements. The controlling firmware can cause the display to scroll through the data in the buffer in response to the operator's pressing various control keys on the keyboard. Scrolling requires only the manipulation of a few control bytes, not the wholesale rearrangement of data in the display buffer.

Alpha Display Control Logic

The Control Logic is the alphanumeric interface between the mainframe and the display. It reads memory via DMA, processes the data, and holds them in a format that the display can use. Each byte of a data word represents either a combination of features to be set or cleared, an ASCII character, or a control code for the display. Figure 24 shows the functions that can be interpreted from each byte.

Data bytes consist of a 7-bit ASCII code and a high-order 0, and they will be interpreted as the corresponding ASCII code unless the foreign character set has previously been chosen. If the high-order bit is set, the five low-order feature bits are latched and held until another feature byte occurs to change the state.

The EOL command fills the remainder of the current line buffer

(one of two local buffers within the CRT) with blanks. The next data byte will be the first character of the next line of displayed characters.

During normal operation, the Control Logic will read the data at address 70000₈, complement them, and use that as the address for the first character to be displayed. From that point on the address will be incremented by 1 for each new data word. The NWA command indicates that the contents of the next address are to be interpreted as the complement of the address for the next data word. The address will then be incremented from the new point.

In addition to being the pointer of the first word of a page, 70000₈ is also used to choose between the alpha mode and the graphics mode. If the high-order bit is a 0 and the graphics hardware is installed, then the display will be the graphics mode.

An example of an alpha mode data pattern is shown in Fig. 25.

As each data byte is processed, the data are placed into a 12-bit word. The first 7 bits contain the ASCII code for the displayed character. The last 5 bits indicate whether any ongoing highlighting should be applied to this character. These feature bits were previously specified by a control code, whereupon they were latched by the Control Logic and were applied to every character until the latches were changed or cleared. These 12-bit words are stored in groups of 80 in one of two local line buffers within the