I've written this flash byte macro for the isd51 demo from Keil. It seem to "work", but returns ACC != 0 indicating an error (Think it's 0xCD). WDT is not in use, as our hardware was designed before this feautre became common. FlashMagic works just fine. I'm ticking the 6clks/sycle box. I've failed to find any messages about simmilar problems. Could it be hardware ? or ? ... Any documentation on the return code(s) from PGM_MTP ?

PGM_MTP EQU 0xFFF0 ; 89C51RD2
XTAL EQU 16 ; Xtal frequency rounded down
sfr AUXR1 =0xA2 ; SFRegister AUXR1 in 89C51RD2
ENBOOT EQU 0x20 ; Bit in AUXR1 to ENable BOOTrom shadow at FC00-FFFF
DISBOOT EQU 0xDF ; Bin in AUXR1 to DISable BOOTrom shadow at FC00-FFFF

CWRITE MACRO
; PUSH IE
; PUSH AR1
; CLR EA ; Dissable all interrupts
ORL AUXR1,#ENBOOT ; Enable shadow BOOTrom at PGM_MTP
MOV DPL,R0 ; DPL <- R0
MOV DPH,A ; DPH <- ACC
MOV R0,#XTAL ; R0 <- #XTAL
MOV R1,#02H ; R1 <- #0x02 { IAP call = "Program Byte". (No WDT) }
MOV A,CBLK ; ACC <- CBLK (IDATA, byte address)
LCALL PGM_MTP ; Returns ACC 0 if o.k. else !0
ANL AUXR1,#DISBOOT ; Disable shadow BOOTrom at PGM_MTP
MOV A,#0
; POP AR1
; POP IE
ENDM

I have a slightly different version of the same called
from "C" that I've used to print the return value.