A0 vendor command can be used to read and write to internal memory of EZ-USB (AN21xx/FX/FX1/FX2/FX2LP). For this vendor command to work the CPU of the device should be in reset. To put the CPU in reset send A0 vendor command with E600 (for FX1/FX2/FX2LP for older parts like FX use 7F92) as value and 01 as data. To bring the CPU out of reset send 00 as data. For reading and writing external memory you can download vend_ax example to the device and then use A3 vendor command. Both these vendor commands use Value field to specify the memory location.