Viewing 8086 memory-areas.pptVIP

Viewing 8086 memory-areas A peek into the video display memory, the real-mode Interrupt Vector Table, and the ROM-BIOS DATA AREA Hexadecimal displays The ability to exhibit computer-data in a form that’s understandable will be vital for exploring (and for debugging) our system The easiest scheme for doing it will be to show binary values in hexadecimal format Let’s look at a straightforward algorithm to convert any 16-bit number into a string of (four) hexadecimal numerals Our ‘ax2hex’ procedure We create an array of the ascii-codes for the sixteen hexadecimal digit-characters hex: .ascii “0123456789ABCDEF” Our procedure expects the binary value to be in register AX and the memory-address for the hex-string is in registers DS:DI Our procedure will preserve the contents of the CPU’s registers (no ‘side-effects’) A 4-bit left-rotation A bitwise ‘AND’ opration Array ‘lookup’ operation Algorithm implementation ax2hex: # converts the word-value in AX to a hex-string at DS:DI pusha # save general registers mov $4, %cx # setup digit-count nxnyb: rol $4, %ax # rotate hi-nybble into AL mov %al, %bl # copy the nybble into BL and $0xF, %bx # isolate the nybble’s bits mov hex(%bx), %dl # lookup nybble’s ascii-code mov %dl, (%di) # store numeral into buffer inc %di # advance the buffer index loop nxnyb # generate remaining digits popa # restore saved registers ret # return control to the caller hex: .ascii “0123456789ABCDEF” # array of hex numerals Algorithm applications We can use this binary-to-hex algorithm to view the contents of two memory-regions which ROM-BIOS startup-code initializes: The table of ‘real-mode’ interrupt vectors The values in the ROM-BIOS DATA-AREA Two ‘boot-sector’ demo-programs are named ‘viewivt.s’ and ‘viewrbda.s’ They are on the CS 630 website: /~cruse/cs630 Direct-Drawing to VRAM Both demo-programs perform their display by writing character-codes directly into the text-mode video