Tandy 200 RAM

From Bitchin100 DocGarden

Jump to: navigation, search

This page captures some useful information for programming with T200 RAM.

Contents

[edit] Bank Selection

T200 RAM is always 24k blocks, from A000 to FFFF. There can be any number of banks from 1 to 3.

In the T200 memory map, there are 6 defined blocks - 3 ROM and 3 RAM. Bank selection is accomplished by writing to IO port D8 (216). The current setting of the bank selection may be read back from D8 also. 

Bit 1,0 --> selection of ROM bank (0000-9FFF or 0000-7FFF) 
00 = main ROM (40k) 
01 = multiplan (32k)
10 = option ROM (if installed)
Bit 3,2 -->  selection of RAM bank (A000-FFFF)
00 = bank 1
01 = bank 2 (if installed)
10 = bank 3 (if installed)

Register D8 can be written at any time, with catastrophic results if not done correctly!

[edit] Bank To Bank Copy

The T200 ROM provides a routine to perform a bank-to-bank memory copy.

To accomplish this, the T200 uses spare memory in the Tandy 200 Timer to store the source and destination ram banks.

Then there are a set of rom routines that do the bank to bank copies.

So, you can imagine then how this works - you store the critical data somewhere accessible always, then bank switch getting all the info you need about the directories, then do the byte copies.

There is a routine at 2FC5 that copies BC bytes from (HL) in source to (DE) in target-- you could use that to copy things to ALTLCD in the ram banks-- then you have your environment prepped for doing the bank to bank copy from RAM.

[edit] Remote block copy code at 2FC5 

; this routine copies BC bytes from (HL) in source to (DE) in target
; enter with source RAM visible

2FC5H  (C5H) PUSH B		
2FC6H  (46H) MOV B,M		; load B with (HL)
2FC7H  (DBH) IN 91H		; get target data
2FC9H  (D3H) OUT D8H		; switch to target
2FCBH  (78H) MOV A,B		; 
2FCCH  (12H) STAX D		; store (HL) in (DE) 
2FCDH  (DBH) IN 90H
2FCFH  (D3H) OUT D8H		; switch to source
2FD1H  (C1H) POP B		
2FD2H  (23H) INX H
2FD3H  (13H) INX D
2FD4H  (0BH) DCX B		; decrement BC
2FD5H  (79H) MOV A,C		; 
2FD6H  (B0H) ORA B
2FD7H  (C2H) JNZ 2FC5H		; loop back until B copies are performed....
2FDAH  (C9H) RET

[edit] ALTLCD

ALTLCD exists in all 3 RAM banks, and is 640 bytes long, running from F7B0 (63408) to FA2F (64047).

Retrieved from "http://bitchin100.com/wiki/index.php?title=Tandy_200_RAM"
Personal tools