在MAXQ8913微控制器中從RAM執(zhí)行應(yīng)用程序

;; Read the banner string from flash and output it over the serial port. Since
;; we are running from RAM, we can read from the flash directly without having
;; to use the Utility ROM data transfer functions (moveDP0inc, etc...).

move SC.4, #0
move DPC, #0 ; Set pointers to byte mode.
move DP[0], #(stringData * 2) ; Point to byte address of string data.

stringLoop:
move Acc, @DP[0]++
sjump Z, stringEnd
lcall #TxChar
sjump stringLoop
stringEnd:
move DPC, #1Ch ; Set pointers to word mode.

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;
;; This portion of the code (addresses 200h and higher) will remain in flash.

org 0200h

stringData:
db 0Dh, 0Ah, Executing code from RAM...., 00h

請(qǐng)注意，如圖1所示，SC.4 (CDA0)位影響將哪一半程序閃存(上半頁或下半頁)以字節(jié)模式映射至數(shù)據(jù)內(nèi)存。當(dāng)使用字模式指針時(shí)，整個(gè)程序閃存被一次性映射至數(shù)據(jù)內(nèi)存。

第二，現(xiàn)在雖然閃存在數(shù)據(jù)空間可存取，但SRAM不可直接存取。這意味著不能對(duì)SRAM的存儲(chǔ)單元進(jìn)行讀或?qū)懖僮?，?yīng)用程序必須采取迂回的方法。從SRAM存儲(chǔ)單元讀取數(shù)據(jù)可按照在閃存中運(yùn)行的代碼從閃存存儲(chǔ)單元讀取數(shù)據(jù)相同的方式實(shí)現(xiàn)―利用應(yīng)用ROM數(shù)據(jù)傳遞函數(shù)(moveDP0inc等)。然而，由于在應(yīng)用ROM中沒有類似的函數(shù)可實(shí)現(xiàn)直接寫操作，所以應(yīng)用程序必須提供一個(gè)小函數(shù)駐留在閃存中，該函數(shù)可被RAM中駐留的代碼直接調(diào)用來執(zhí)行寫操作。

以下的代碼演示用來讀和寫RAM變量varA的方法，其初始值隨其它部分的應(yīng)用程序被從閃存復(fù)制到RAM，地址范圍為0000h-01FFh。

scall printVar
scall incrVar
scall printVar
scall incrVar
scall printVar
scall incrVar

move Acc, #0Dh
lcall #TxChar
move Acc, #0Ah
lcall #TxChar

sjump $

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;
;; Variables stored in RAM (program) space. They can be read using the
;; Utility ROM data transfer functions (such as UROM_moveDP0) and written
;; using the writeDP0 function which remains in flash.
;;

varA:
dw 'A'

;================================================================
;=
;= printVar
;=
;= Reads the varA RAM variable value and sends it over the serial port.
;=

printVar:
move DPC, #1Ch ; Word mode
move DP[0], #varA ; Variable's location in UROM data space
lcall UROM_moveDP0 ; Moves variable value into GR.
move Acc, GR
lcall #TxChar
ret

;==============================================================
;=
;= incrVar
;=
;= Reads the varA RAM variable value, adds 1 to it, and stores it back in RAM.
;=

incrVar:
move DPC, #1Ch ; Word mode
move DP[0], #varA ; Variable's location in UROM data space
lcall UROM_moveDP0 ; Moves variable value into GR.

move Acc, GR
add #1
move GR, Acc
lcall writeDP0

ret

;==================================================================
;=
;= TxChar
;=
;= Outputs a character to the serial port.
;=
;= Inputs : Acc.L - Character to send.
;=

org 01F0h
move SBUF, Acc ; Send character.
TxChar_Loop:
move C, SCON.1 ; Check transmit flag.
sjump NC, TxChar_Loop ; Stall until last transmit has completed.
move SCON.1, #0 ; Clear the transmit flag.
ret

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;
;; This portion of the code (addresses 200h and higher) will remain in flash.

org 0200h

stringData:
db 0Dh, 0Ah, Executing code from RAM...., 00h

;===============================================================
;=
;= WriteRAM
;=
;= This is a routine that can be called by code running in the RAM to load
;= a new value into a byte or word location in the RAM.
;=
;= Inputs : DP[0] - Location to write (absolute starting at 0000h) in RAM.
;= GR - Value to write to the RAM location.
;=
;= Notes : DP[0] must be configured to operate in word or byte mode as
;= desired before calling this function. Following a call to this
;= function, DP[0] must be refreshed before it is used to read data.
;=

writeDP0:
move @DP[0], GR
ret

在執(zhí)行時(shí)，示例代碼通過串口輸出以下的文字(圖2)。

圖2. 示例代碼通過串口的輸出文字

結(jié)論
利用MAXQ8913及其它MAXQ微控制器采用的Harvard內(nèi)存映射架構(gòu)，可以將不同的物理內(nèi)存段(例如數(shù)據(jù)SRAM)映射為程序或數(shù)據(jù)內(nèi)存空間。在數(shù)據(jù)SRAM中執(zhí)行部分應(yīng)用程序?yàn)樾阅芴嵘徒档凸奶峁┝藵摿?。不過該過程也增加了應(yīng)用程序的復(fù)雜性。