--- /dev/null
+;
+; Extended memory driver for the GEORAM cartridge
+;
+; Ullrich von Bassewitz, 2002-11-29
+;
+
+ .include "zeropage.inc"
+
+ .include "em-kernel.inc"
+ .include "em-error.inc"
+
+
+ .macpack generic
+
+
+; ------------------------------------------------------------------------
+; Header. Includes jump table
+
+.segment "JUMPTABLE"
+
+; Driver signature
+
+ .byte $65, $6d, $64 ; "emd"
+ .byte $00 ; EM API version number
+
+; Jump table.
+
+ .word INSTALL
+ .word DEINSTALL
+ .word PAGECOUNT
+ .word MAP
+ .word MAPCLEAN
+ .word COPYFROM
+ .word COPYTO
+
+; ------------------------------------------------------------------------
+; Constants
+
+GR_WINDOW = $DE00 ; Address of GEORAM window
+GR_PAGE_LO = $DFFE ; Page register low
+GR_PAGE_HI = $DFFF ; Page register high
+
+; ------------------------------------------------------------------------
+; Data.
+
+.data
+
+pagecount: .word 2048 ; Currently fixed
+
+.code
+
+; ------------------------------------------------------------------------
+; INSTALL routine. Is called after the driver is loaded into memory. If
+; possible, check if the hardware is present and determine the amount of
+; memory available.
+; Must return an EM_ERR_xx code in a/x.
+;
+
+INSTALL:
+ lda #<EM_ERR_OK
+ ldx #>EM_ERR_OK
+ rts
+
+; ------------------------------------------------------------------------
+; DEINSTALL routine. Is called before the driver is removed from memory.
+; Can do cleanup or whatever. Must not return anything.
+;
+
+DEINSTALL:
+ rts
+
+
+; ------------------------------------------------------------------------
+; PAGECOUNT: Return the total number of available pages in a/x.
+;
+
+PAGECOUNT:
+ lda pagecount
+ ldx pagecount+1
+ rts
+
+; ------------------------------------------------------------------------
+; MAP: Map the page in a/x into memory and return a pointer to the page in
+; a/x. The contents of the currently mapped page (if any) are assumed to be
+; dirty and must be saved into secondary storage if this is necessary.
+;
+
+MAP: sta tmp1
+ txa
+ asl tmp1
+ rol a
+ asl tmp1
+ rol a
+
+ sta GR_PAGE_HI
+ lda tmp1
+ lsr a
+ lsr a
+ sta GR_PAGE_LO
+
+ lda #<GR_WINDOW
+ ldx #>GR_WINDOW
+ rts
+
+; ------------------------------------------------------------------------
+; MAPCLEAN: Map the page in a/x into memory and return a pointer to the page
+; in a/x. The contents of the currently mapped page (if any) are assumed to
+; be clean, so if this is an advantage for the driver, the current contents
+; may be discarded.
+
+MAPCLEAN = MAP ; Identical for GEORAM
+
+
+; ------------------------------------------------------------------------
+; COPYFROM: Copy from extended into linear memory. A pointer to a structure
+; describing the request is passed in a/x.
+; The function must not return anything.
+;
+
+COPYFROM:
+ jsr setup
+
+; Setup is:
+;
+; - ptr1 contains the struct pointer
+; - ptr2 contains the linear memory buffer
+; - ptr3 contains -(count-1)
+; - tmp1 contains the low page register value
+; - tmp2 contains the high page register value
+; - X contains the page offset
+; - Y contains zero
+
+ jmp @L5
+
+@L1: lda GR_WINDOW,x
+ sta (ptr2),y
+ iny
+ bne @L2
+ inc ptr2+1
+@L2: inx
+ beq @L4
+
+; Bump count and repeat
+
+@L3: inc ptr3
+ bne @L1
+ inc ptr3+1
+ bne @L1
+ rts
+
+; Bump page register
+
+@L4: inc tmp1 ; Bump low page register
+ bit tmp1 ; Check for overflow in bit 6
+ bvc @L6 ; Jump if no overflow
+ inc tmp2
+@L5: lda tmp2
+ sta GR_PAGE_HI
+@L6: lda tmp1
+ sta GR_PAGE_LO
+ jmp @L3
+
+; ------------------------------------------------------------------------
+; COPYTO: Copy from linear into extended memory. A pointer to a structure
+; describing the request is passed in a/x.
+; The function must not return anything.
+;
+
+COPYTO:
+ jsr setup
+
+; Setup is:
+;
+; - ptr1 contains the struct pointer
+; - ptr2 contains the linear memory buffer
+; - ptr3 contains -(count-1)
+; - tmp1 contains the low page register value
+; - tmp2 contains the high page register value
+; - X contains the page offset
+; - Y contains zero
+
+ jmp @L5
+
+@L1: lda (ptr2),y
+ sta GR_WINDOW,x
+ iny
+ bne @L2
+ inc ptr2+1
+@L2: inx
+ beq @L4
+
+; Bump count and repeat
+
+@L3: inc ptr3
+ bne @L1
+ inc ptr3+1
+ bne @L1
+ rts
+
+; Bump page register
+
+@L4: inc tmp1 ; Bump low page register
+ bit tmp1 ; Check for overflow in bit 6
+ bvc @L6 ; Jump if no overflow
+ inc tmp2
+@L5: lda tmp2
+ sta GR_PAGE_HI
+@L6: lda tmp1
+ sta GR_PAGE_LO
+ jmp @L3
+
+; ------------------------------------------------------------------------
+; Helper function for COPYFROM and COPYTO: Store the pointer to the request
+; structure and prepare data for the copy
+
+setup: sta ptr1
+ stx ptr1+1 ; Save passed pointer
+
+; Get the page number from the struct and adjust it so that it may be used
+; with the hardware. That is: lower 6 bits in tmp1, high bits in tmp2.
+
+ ldy #EM_COPY_PAGE+1
+ lda (ptr1),y
+ sta tmp2
+ dey
+ lda (ptr1),y
+ asl a
+ rol tmp2
+ asl a
+ rol tmp2
+ lsr a
+ lsr a
+ sta tmp1
+
+; Get the buffer pointer into ptr2
+
+ ldy #EM_COPY_BUF
+ lda (ptr1),y
+ sta ptr2
+ iny
+ lda (ptr1),y
+ sta ptr2+1
+
+; Get the count, calculate -(count-1) and store it into ptr3
+
+ ldy #EM_COPY_COUNT
+ lda (ptr1),y
+ eor #$FF
+ sta ptr3
+ iny
+ lda (ptr1),y
+ eor #$FF
+ sta ptr3+1
+
+; Get the page offset into X and clear Y
+
+ ldy #EM_COPY_OFFS
+ lda (ptr1),y
+ tax
+ ldy #$00
+
+; Done
+
+ rts
+
+
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