The System Manual of SOL-20 - History of Computers

V. SUBROUTINES (cont.)
Second are the defined register usages when interfacing at the
machine language level with SOLOS.
Whenever a machine program is executed by SOLOS (via the EXEC or
XEQ command, or via a custom command), the stack pointer and HL
registers are predefined by SOLOS >. The stack pointer is set
such that the user may perform stacking operations which will
use the SOLOS stack. The SOLOS stack begins at the end of the
SOLOS RAM area and works its way down from there. Excessive use
of this stack can destroy data maintained by SOLOS within its
RAM area. The stack is also prepared so that the user may issue
a standard RET instruction to return control to SOLOS command
mode processor.
The HL register pair is initialized to point to the very beginning
of SOLOS. It is at this point that the SOLOS jump table begins.
The user program may then use the address presented in the HL
register pair as the beginning of the jump table.
This address is provided for two reasons:
1. CUTER may be located at any address in memory, providing the
means for programs to function with CUTER located at any
address, and
2. the first byte of the jump table for SOLOS is different from
the first byte for CUTER, providing an easy means of distinguishing
between SOLOS and CUTER.
Third is the SOLOS jump table (see next page). All requests to SOLOS
should be made based on this jump table and not to the actual routine
addresses as scattered throughout SOLOS. By using only this jump
table, the user can be assured of maintaining compatibility between
SOLOS and CUTER.
15