;
-; Support code for swap device activity
+; Support code for mmio RAM based swap devices used as if they were
+; banked memory. We could have made them swap but its faster to treat
+; them as memory and do switching and exchanges. It also allows us to
+; avoid the cost of having an empty space as needed for swap out and
+; then swap in. On a PX4 with a single 32K cartridge that is critical.
+;
+; The key element here is the map translation table. Each of the
+; logical banks has a 1 byte entry that gives its current location
+; as either
+; 1 - RAM
+; 2 - Cartridge, low 32K
+; 3 - Cartridge, high 32K (if present)
+; 0x80-0x83 - Sidecar in 32K chunks
+;
+; When we "switch" to a bank we actually swap the RAM contents with
+; the I/O memory that held the bank we switched to, and then we fix
+; up the table. That makes all the magic going on conveniently
+; invisible to the core code.
;
.globl _cartridge_copy
.globl _map_translate
.globl _map_live
+ .globl _read_from_bank
+
+ .globl _romd_addr
+ .globl _romd_off
+ .globl _romd_size
+ .globl _romd_mode ; byte size
+
+ .globl map_process_save
+ .globl map_kernel_restore
+
+ .module swapdev
+
+ .include "kernel.def"
.area _COMMONMEM
+;
+; Copy the RAM into the given bank in the cartridge. This is used when
+; we do a fork.
;
; A holds the bank to use (1 = RAM 2/3 for cartridge 128+ sidecar
;
ret
;
-; Exchange the process in bank a with the process in RAM
+; Exchange the process in bank a with the process in RAM. This is
+; used when we do a task switch between two processes.
;
_cartridge_exchange:
push bc
.area _CODE
-
+;
+; Compute the size of a cartridge. We can only use 32K sized chunks,
+; so the 16K cartridge while detected is useless.
+;
_cartridge_size:
ld c, #0xA512
xor a
.area _COMMONMEM
+;
+; A holds the sidecar bank to use with bit 7 set. We call this
+; function when we are forking a process so that we can write a copy
+; of RAM into the other bank (where it will become the parent)
+;
_sidecar_copy:
push bc
push de
pop bc
ret
+;
+; Exchange RAM and the process in sidecar bank A (with bit 7 set).
+; Used when we are task switching. This is rather ugly as the
+; sidecar memory helpfully auto-increments the low byte!
+;
_sidecar_exchange:
push bc
push de
ret
;
-; Make the requested process the current one in memory. We use this
-; on task switching but also on the floppy I/O when swapping in or
-; out. Preserves DE.
+; Make the requested bank the current one in memory. We use this
+; when task switching. It's the PX4 logical equivalent of the out
+; instructions on saner ports that switch context. It must be called
+; on a private stack as it will exchange the kernel/irq stacks of the
+; old and new task as it runs.
;
_make_mapped:
ld c, a
in_memory2:
pop de ; recover task pointer
ret
+
+;
+; We have to deal with the awkward case of I/O wanting to access other
+; banks for swapping. On a normal setup this is fairly trivial, in our
+; case the memory they want to copy may in fact be stuffed into an I/O
+; mapped device so we need to do some gymnastics.
+;
+_read_from_bank: ; (page, dptr, buf) always 128 bytes
+ ld de, (_romd_mode)
+ ld d, #0
+ ld hl, #_map_translate
+ add hl, de
+ ld a, (hl) ; Translation entry
+ cp #1
+ jr z, memory_read ; It is in RAM
+ ;
+ ; Turn virtual address into offset in swap in HL as both
+ ; other methods need this
+ ;
+ ld hl, (_romd_off) ; in bytes, userspace ptr
+ ld de, #SWAP_VTOP
+ or a
+ sbc hl, de ; convert to zero based
+ bit 7, a ; cartridge is 2/3
+ jr z, cartridge_read
+ ;
+ ; Pull it from the sidecar (0x80-0x83)
+ ;
+ ld b, #0
+ rra ; effectively multiply by 32768
+ rr b ; 0x0 or 0x80
+ and #0x3f ; 32K per process
+ out (0x92), a ; select right 64K bank
+ ;
+ ; Now work out which 256 byte page is needed
+ ;
+ ld a, h
+ add b ; 32K low bit of bank number
+ out (0x91), a ; middle byte of I/O offset
+ ld a, l
+ out (0x90), a ; low byte of I/O offset
+ ld bc, #0x8093 ; 128 count, port 92)
+ ld hl, (_romd_addr)
+ inir ; 128 bytes into hl
+ ret
+;
+; 2 is the low 32K, 3 the high
+;
+cartridge_read:
+ cp #2
+ jr z, cart_read_1
+ set 7,h ; offset by another 32K
+cart_read_1:
+ ld a, h
+ out (0x10), a ; set high byte
+ ld de, (_romd_addr)
+ ld b, #0x80 ; 128 bytes
+cart_read_l:
+ ld a, l ; sufficient as this won't
+ out (0x11), a ; cross a page
+ in a, (0x12)
+ ld (de), a
+ inc de
+ inc l
+ djnz cart_read_l
+ ret
+;
+; Pull 128 bytes from user space. We pull a trick here.
+; We are copying between user space and kernel writable data.
+; Only R/O space is banked on the PX4 systems so we don't need
+; to play bank pogo.
+;
+memory_read:
+ ld bc, #0x80
+ ld de, (_romd_addr)
+ call map_process_save
+ ldir
+ call map_kernel_restore
+ ret
+
+_map_translate: .ds MAX_MAPS
+_map_live: .dw 0
public / FUZIX.git / commitdiff