CAC 2021-12-10

Reverse engineering DN6600 memory management

*  setptw
*         converts an 18 bit absolute address to a 15 bit virtual
*         address and sets up the page table entry in the cpu page
*         table.  this routine is only required if more than 32K of
*         memory is configured for a dn6670.  a 'tra -1' is stored into
*         setptw+1 by init if otherwise.
*  input:
*         a reg - 18 bit absolute address
*  output:
*         a reg - 15 bit virtual address
*  modified registers: none
          rem                 a 'tra -1,*' is stored in setptw+1 by init if
          rem                 only 32k is configured
setptw    subr      ptw,(inh,x2)        cannot allow interruptions
          ldx2      a.v001-*,*          .crpte
          tsy       setpte-*
          ora       l.v001-*  concatenate with window address
          return    setptw    all done

a.v001    ind       .crpte
l.v001    vfd       18/window
          rem       the subr macro should be the first statement in a
          rem       subroutine. it creates a zero word to store the
          rem       address into, some register save instructions
          rem       and it contains the register restore instructions.
          rem       the first argument to the macro should be a unique
          rem       3 character (or less) string. this character string
          rem       will be used in building the names of places to store
          rem       specified registers so that the stored registers may
          rem       be referenced by the subroutine. names are of the form
          rem       xxxsn where xxx is the 3 character string, the letter
          rem       s, and n where n is the register name, x1, x2, x3, a
          rem       q, or i (indicators). the registers to be saved
          rem       are specified in parens as the second argument. if
          rem       the character string 'inh' is one of the names, a
          rem       sti followed by an inh will be the first two instr-
          rem       uctions generated and the last will be an ldi before
          rem       returning to the caller.
000475                cptp   set     317             cpu page table pointer
000701                .crcpt set     449             address of cpu page table
000702                .crpte set     450             address of variable cpu page table entry

window    bool    77400    base address of paged memory
bwndow    bool    77000    base address of buffer window
*  setpte
*      common subroutine used to set a page table entry
*  input:
*      a reg - 18 bit absolute address
*      x2    - address of page table entry
*  output:
*      a reg - low-order 8 bits of virtual address (offset in page)
setpte    subr      spt
          sta       sargsv-*  save to provide offset
          iana      -256      get page number
          iora      pte.a     turn on active bit
          sta       0,2       put in relevant page table entry
          lda       sargsv-*  get page offset
          iana      255
          return    setpte

pte.r     bool      200       page table entry read only bit
pte.s     bool      100       page table entry security bit
pte.a     bool      40        page table entry active bit
          lda       l.t002-*  (=32768) yes. calculate last address in lower 32k
          iaa       -1
          sta       istpcl-*  stops memory clear loop
          iaa       -256      account for paging window
          iaa       -256      and buffer window
          sta       a.t007-*,*          (mvplmm) set lower memory maximum address
*  check pager operation
          ldx1      l.t009-*  get address of loc. 0
          lda       0,1       save its contents
          sta       itloc0-*
          ila       -1        put something recognizable there
          sta       0,1
          ldx1      l.t004-*  (window) base of window
          stz       0,1       clear test cells
          stz       -256,1
          ldx3      a.t009-*,*          (.crpte)
          lda       l.t006-*  (=o100040) init page table to window page 77000
          sba       l.t003-*  (=o400)
          sba       l.t003-*  (=o400)
          sta       0,3
          lda       a.t006-*,*          (.crcpt) init cpu pager
          sta       a.t004-*,*          (cptp)
          ila       -1        lets see where this goes
          sta       0,1       store test value
          stz       0,3       disable pager
          lda       0,1       this is the real 77400, has it changed?
          tnz       itl120-*  yes. bad news
          lda       -256,1    this is where store should go
          icmpa     -1        is it correct?
          tnz       itl120-*  no. bad news

l.t002    dec       32768
istpcl    oct       0
a.t007    ind       mvplmm    lower memory maximum address (in utilities)
l.t009    oct       0
itloc0    bss       1
l.t004    vfd       18/window
a.t009    ind       .crpte
l.t006    vfd       10/128,3/1,5/0
l.t003    oct       400
a.t006    ind       .crcpt
a.t004    ind       cptp
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