Elf/OS

Hardware:

1802 based computer, built equivalent to Micro/Elf, Elf, Elf II, or Super Elf
Serial Interface equivalent to Giant Board (Q and EF4)
IDE/CF interface
Minimum of 12k ram
Optional memory mapper

Ports:

Port	In	Out
1	Activate Pixie graphics¹	Deactivate Pixie graphics¹
2		Write disk controller selection port³
3	Read disk controller selected port³	Write disk controller selected port³
4	Read input switches/keypad¹	Write data displays¹
5		Select memory pager cell to change³
6		Write new memory offset to selected pager cell³
7	Read input ASCII keyboard²	Write output port²

¹ Defined by standard Elf system documentation
² As per Giant Board documentation
³ Non-standardized ports used by my hardware

EF Lines

EF1	Pixie graphics frame indicator
EF4	Serial IN line

Memory Pager:

This memory mapper works on the principal of the top 4 address lines being used to access an additional 8 bits of address space stored in a 16x8 memory. This brings the total address bits to 20 providing 1MB of ram segmented into 256 4k pages.

The circuit for the memory mapper ignores position 0, thus allowing during reset the bottom 4k of ram to be available.

Memory Map:

This map represents the target memory map for Elf/OS.

0000 007F	Scratchpad area
0080 00FF	Keyboard input buffer
0100 02FF	Disk transfer buffer
0300 1FFF	Elf/OS Kernel
2000 EFFF	Space for user programs
F000 FFFF	Bios ROM

Register Assignments:

R2	Stack Pointer
R3	Main PC
R4	Standard Call Vector
R5	Standard Return Vector
R6	Used by SCALL and SRET routines