Software algorithm to test sram on microcontroller at high speed

I have an 8051 microcontroller with external ram attached to it and I want to test the ram to see that it can store and load data correctly (which I think it currently does not).

In my program, I have it where the value 80h is written to 1st byte of memory and the value increments as each memory address is incremented but the value rolls over to zero every 256 bytes starting with the 128th byte.

This is my function:

;extended memory test. C=1=memory fault
memtest:
  mov DPTR,#0h
  mov R7,#80h
  ;fill with incrementing values
  post1:
    mov A,R7
    movx @DPTR,A
    inc R7
    inc DPTR
    mov A,DPH
    orl A,DPL
  jnz post1
  ;Test for incrementing values
  mov DPTR,#0h
  mov R7,#80h
  post2:
    clr C
    movx A,@DPTR
    subb A,R7
    setb C
    jnz isbadram
    movx @DPTR,A
    mov A,R7
    inc R7
    inc DPTR
    mov A,DPH
    orl A,DPL
    clr C
  jnz post2
  isbadram:
ret

But I think I need to do more of a rigorous test. I think I need to write and read different values multiple times to the same locations but I don't know how many times to test a location and I don't know the best values to try.

I'm just afraid if I test a location too many times, then my program will spend forever testing memory before it begins doing anything useful.

Luckily I own some microcontrollers that can execute instructions at about 40nS each. I have others of the same family that execute instructions at about 0.5uS each.

So whats the best way to test a ram chip already installed in the system without spending too much time on the testing?