Leo Paul wrote:
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...a page or two of external RAM available in your system

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why??
if one assumes acc ,psw registers are ok
then for a simple write-read back these
are sufficient
mov a,#055h
mov 00,a
clr a
mov a,00
cjne a,#055h,fault
mov a,#0aah
mov 00h,a
clr a
mov a,00h
cjne a,#aah,fault
mov psw,#00h
mov r0,#01h;r0 has been verified

loop:
mov @r0,#055h
cjne @r0,#055h,fault
mov @r0,#0aah
cjne @r0,#0aah,fault
inc r0
cjne r0,#00h,loop

regards
pranav