Memotech Limited MTX 500 User's Guide download pdf (Page 14)

CASSETTE TAPE

By default REMEMOrizer does not load from or save to cassette tape. Almost all of the Memotech

library on cassette has been converted into .MTX file format. Instead, REMEMOrizer supports "virtual

cassette tapes”.

"Virtual cassette tapes" are implemented as hidden areas of SRAM, per the diagram above. There are

two virtual tapes, 64KB each.

Looking at the known library of Memotech cassettes, almost all of them will fit within 64KB.

The REMEMOrizer supplied OS ROM is patched to jump to virtual tape code at the end of SDX ROM

5. The last portion of this ROM is in fact writeable, and is where the virtual tape logic keeps its

variables.

Virtual cassette tapes are accessed from CP/M using the REZTAPE command covered in Chapter 5.

(SW2 on the GODIL allows the user to switch between “Virtual” tape support and physical tape

support. Refer to Chapter 3, Installation, to identify the position of switch SW2.)

FLOPPY DISC CONTROLLERS

REMEMOrizer includes an SD Card. SD Cards between 64MB and 1GB are supported. Only 64MB of

data may be stored on them. REMEMOrizer considers them to contain 8 8MB partitions. This is

somewhat generous, as the entire Memotech software library will fit comfortably within one 8MB

partition.

It accesses this using a Serial Peripheral Interface (SPI) interface. It has hardware support for driving

the SPI interface so that byte transfer speed is effectively limited by the Z80. It has a novel feature in

that reading data from SPI on one port triggers the sending of an 0xff byte to trigger the next

transfer. The means that reading of data from SD Card needn't be twice as slow as writing it (as it

would otherwise be).

Compared to REMEMOTECH, REMEMOrizers SD Card is slow. REMEMOTECH can transfer one

byte in 21T cycles (at upto 25MHz), because the SPI logic is driven at 50MHz. REMEMOrizer can

transfer one byte in in 28T cycles (at 4MHz), because the SPI logic has to be driven from the CPU

clock. Having said this, in a simple speed trial REMEMOrizer is still over 4x faster than SDX floppy,

and massively faster than cassette tape.

Unfortunately the fact that CP/M sectors are 128 bytes and SD Card blocks are 512 bytes makes the

whole thing somewhat inefficient. To read a 128 byte sector, we must read the enclosing 512 byte

block. And to write a 128 byte sector, we must read the enclosing 512 byte block, modify a part of it,

then write it back. Even with this handicap, it is still usable. Clever driver software helps improve

things.

A green LED flashes when SD Card is being accessed and for a couple of seconds afterwards,

and the intent is that the user doesn't remove the SD Card until the LED goes off. This simple

feature allows the SD Card driver code to go faster.

1 2 ... 9 10 11 12 13 14 15 16 17 18 19 ... 32 33

No comments

Memotech Limited MTX 500 User's Guide Page 14