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Register parameters

RST 20H, DEFB $4E
IN:
     A = memory mask arguments (several arguments OR'ed together):

          Destination segment (allocated memory addressed for segment):
          MM_S0 ($00), segment 0
          MM_S1 ($40), segment 1
          MM_S2 ($80), segment 2
          MM_S3 ($C0), segment 3

          Source of memory (to be allocated):
          MM_FIX ($02), fixed workspace (not subject to swapping)
          or
          MM_SLT ($04), explicit slot usage, mask also following:
               MC_CI ($10), internal memory
               MC_C1 ($01), card 1
               MC_C2 ($02), card 2
               MC_C3 ($03), card 3

          Allocation variations:
          MM_EXC ($10), exclusive use of bank
          MM_MUL ($20), use multiple banks

     BC = 0, always

OUT, if call successful:
     Fc = 0
     IX = memory pool handle
OUT, if call failed:
     Fc = 1
     A = error code:
          RC_NA ($06), handle not available
          RC_ROOM ($07), no room

Registers changed after return:
     A.FBCDEHL/..IY same
     .F......./IX.. different 

Notes

MM_EXC and MC_CI use the same argument bit. MM_EXC isn't implemented.

If MM_SLT is specified, C holds bank number ($20-$ff) or slot (0-3).

If only destination segment is specified, you get memory from bank with
most free space.

OS_Mop doesn't allocate any memory, so even the very first OS_Mal can
fail.

If you specified MM_MUL, you can force slot 1-3 during first
OS_Mal by loading A with slot number

If you didn't specify MM_MUL, all memory allocated from that pool is
guaranteed to be in same bank. Additionally, memory area returned from
OS_Mal is guaranteed to be in single page.

If only the memory segment mask are specified, it is probably default'ed to MM_MULMM_FIX is probably an internal usage.

Related calls

OS_Bix, Bind in extended address
OS_Box, restore bindings after OS_Bix
OS_Fc, select fast code (fast bank switching)
OS_Mal, allocate memory
OS_Mcl, close memory (free memory pool)
OS_Mfr, free (previously allocated block) memory
OS_Mgb, get current bank binding in segment
OS_Mpb, set new bank binding in segment
OZ_Mgb, get current bank binding in segment
OZ_Mpb, set new bank binding in segment

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