Zenith Z-100 Series, Technical Manual

Page: 441 / 493

Page 10.15
Z-DOS INITIALIZATION SEQUENCE
The system ROM loads in the loader at address 0:400. The
contents of the instruction pointer (IP) is therefore 400, and
the code segment (CS) register is 0. The other registers are
assumed to contain random data. The loader assumes that
the system ROM read is enough of track zero so as to have
at least the first sector of the director already in RAM. This
may be anywhere from 4 sectors to 17 sectors, depending
on the disk format. This allows the loader to be smaller, since
it does not have to read in the directory. (See Pictorial 10-2.)
TOP OF SYSTENI MEMORY
— LOADER CODE
0:400 (1K)
— 0;0 FIRST MIENIORY ADDRESS
Pictorial 10-2
Loader
The first thing the loader does is to relocate itself. This is
because it must load IO.SYS at address 40:0. The loader
relocates itself to address 400:400 and performs a long jump
to this a current IP. (See Pictorial 10-3.)
TOP OF SYSTEM MEMORY
LOADER CODE
— 400;400 (17K)
— OR IG IONAL LOADER CODE
— 0:400 (IKI
— 0.0 FIRST MEMORY ADDRESS
Pictorial 10-3
Relocation of Loader