Intel 210200-002 manual Language

APPLICATION EXAMPLES

to illuminate dots on the CRT screen. The 2716 character generator makes it possible to display special symbols for word processing or industrial control applications, or to display characters and words in a foreign

.language.

DMAed, and uses a repeated Load String (REP LODS) to DMA 40 words (80 bytes) to the 8276. The routine then checks to see if it is at the bottom of the screen memory, updates the character pointer in memory, restores the registers, and returns from the interrupt.

Screen Memory Feature

One special feature of this design is the iAPX 88's Load String (LODS) instruction to emulate DMA. This DMA function fills the 8276's row buffers which must receive 80 characters (one row on the CRT screen) every 617 microseconds. This is done using an interrupt routine which saves the registers to be used, points to the first character to be

NMI	BRDY
8088	8276

DMA Emulation '

The LODS instruction actually moves each byte of data from memory to the 8276 in one machine cycle by using a special decoding trick to generate both a read signal to memory and a write signal to the 8276. The address decoding is set up so that the screen memory is at memory locations 30H to 7FFH. This memory is also accessed by memory addresses 1030H through 17FFH.

11.34 mHZ

..-....1-__1....-...,

VERTICAL

SYNC

DECODER

CRT

CONTROLLER

VIDEO TO CRT

L.-_.,........---J CCo 6

o

8253·5

COUNTER/8251A

TIMERUSART

t

SERIAL

COMMUNICATIONS

CHANNEL

HORIZ. __ SYNC

FROM 8253

PROGRAM/ 8255A·5 I DISPLAY KEYBOARDj

MEMORY

CONTROllER .

IKEYBOARD I ISTATUS

Figure 4-8.CRT Controller Block Diagram

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