Apple Lisa - Loading the MMU Registers

Confidential
Lisa Hardware Reference Manual
LOADING THE MMU REGISTERS
To set up the MMU, the Operating System must initialize the SORG and
SLIM registers of each of the 128 segments. To set these registers,
the memory map must be disabled.
The MMU can be disabled by setting the SETUP bit. SETUP is set at
power on ~en the map registers are uninitialized, and whenever the
map registers are being loaded. When the memory map is disabled, the
Special I/O address space is enabled, and only the MMU registers and
boot ROM can be acessed. In this state the system hardware decodes
addresses from the 68000 in the following manner:
SPECIAL I/O ADDRESS DECODE
7-Jul-81
23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
t ---------------·--~~--~I--+I----------,-------~-------- I ~-----_+
I segment number Ix IR 1M I ROM address IS I x x xl
+--------·--·-----------+--r~I~~---~---------- I ----------+
Bit 16 is ignored. Bit 14 (M) turns the memory map on (1) or off
(0). If the map is off, bit 15 (R) selects either the system ROM
(0), or an MMU register ~1). If the ROM is selected, the low 14 bits
are the address in ROM. If the HMO is selected, the high order bits
are the segment number of the register to be accessed and bit 3
determines whet~r the Segment Origin register (f), or the Segment
Limit register <B is accessed. Bits 0 - 2 are unused.
When the system.is powered on, the SETUP bit is set. The power on
vectors at address 0 are fetched from ROM and can branch to the boot
program
in the ROM. The boot program can be addressed by any segment
number.
It is recommended that segment 127 be programmed to decode
special I/O and that the boot program jump to segment 127 when it
begins execution. The boot program can initialize the MMU registers
so that context 0 addresses RAM with the low 16 segments and system
I/O with segment 126. Since this makes all the address space in the
system accessible, the system can act as though no memory map were
present. The boot program can then turn off the SETUP bit, but the
program can continue to run from ROM because segment 127 has been
programmed to
special I/O. The Operating System can then be loaded
into RAM and begin normal execution.
When the Operating System wants to load the MMU registers, the
program it uses must be located at a logical address that has bit 14
set. If bit 14 is set and the program is in supervisor mode with the
SEG bits set correctly, the SETUP bit can be set and the program
continues to execute out of RAM as long as the address emitted has
bit 14 set. Whe~ the actual write to the HMO register occurs, bit 14
must be 0 and bit 15 must be set. Once the MHO has been set up, the
SETUP bit must be cleared to return to normal operation. The SETUP
bit can be set by a read or write at the address $00E010 in I/O
space, and reset by a read or write at $00E012.
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