Maxim-integrated MAXQ622 Manual do Utilizador descarregar pdf (Página 245)

MAXQ612/MAXQ622 User’s Guide

Maxim Integrated 17-9

17.5 ROM Example 2: Calling A Utility ROM Function Indirectly

The second example shows the indirect addressing method (lookup table) for calling utility functions. We use the

same function (UROM_MoveDP1Inc) to read our static string, but this time we must figure out the address we want

dynamically. Note the inserted code where we before had a direct call to the function. Also note that the function index

of moveDP1inc is 7.

Text:

DB “Hello World!”,0 ; Define a string in code space.

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;; Function: PrintText

;; Description: Prints the string stored at the “Text” label.

;; Returns: N/A

;; Destroys: ACC, DP[1], DP[0], and GR.

;; Notes: This function assumes that DP[0] is set to word mode,

;; DP[1] is in byte mode, and the device has 16-bit

;; accumulators.

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

PrintText:

move DP[1], #Text ; Point to the string to display.

move ACC, DP[1] ; “Text” is a word address and we need a

sla ; byte address, so shift left 1 bit.

or #08000h ; Code space is mapped to 8000h when running

move DP[1], ACC ; from the ROM, so the address must be masked.

PrintText_Loop:

;

; Fetch the byte from code space.

;

move DP[0], #0800Dh ; This is where the address of the table is stored.

move ACC, @DP[0] ; Get the location of the function table.

add #7 ; Add the index to the moveDP1inc function.

move DP[0], ACC ; Point to where the address of moveDP1 is stored.

move ACC, @DP[0] ; Retrieve the address of the function.

call ACC ; Execute the function.

move ACC, GR

jump Z, PrintText_Done ; Reached the null terminator.

call PrintChar ; Call a routine to output the char in ACC

jump PrintText_Loop ; Process the next byte.

PrintText_Done:

ret

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