Hey John,
After I sent the original email, I realized this also. Instead of
forcing the calling routine to restore HL, I would instead code it using
XTHL/RET vs. PCHL:
LXI D, branch_offset ; Calculated relative to the DAD D opcode below
CALL WHEREAMI ; 31E9H or 3F3DH. HL now contains address of
DAD D opcode on next line
DAD D ; Perform relative address calculation
XTHL ; Restore HL, push jump address to stack
RET ; Jump to new relative location
But this still uses codes that are not allowed in a BASIC program.
Ken
On 6/1/18 11:06 AM, John R. Hogerhuis wrote:
Very nice.
Seems like this destroys DE and HL.
You have to sacrifice some register as the relative offset, I guess.
As a small improvement, since the code at WHEREAMI is
XTHL
PCHL
it is actually preserving the original HL on the stack before it
"returns".
So, you could remove the POP D and instead have a POP H at the jump
target, so the jump target executes with the original HL value.
Then you can keep HL intact for program logic.
LXI D, branch_offset ; Calculated relative to the DAD D opcode below
CALL WHEREAMI ; 31E9H or 3F3DH. HL now contains address of
DAD D opcode on next line
DAD D ; Perform relative address calculation
PCHL ; Jump to new relative location
TARGET:
POP H ; Pop old HL value from Stack
; ...
-- John.
On Fri, Jun 1, 2018 at 12:15 AM, Ken Pettit <[email protected]
<mailto:[email protected]>> wrote:
Hi John,
So based on Ron's research, the WHEREAMI function in ROM can be
used as another way to perform a relative branch without the need
for a fixed location in RAM:
LXI D, branch_offset ; Calculated relative to the DAD D
opcode below
CALL WHEREAMI ; 31E9H or 3F3DH. HL now contains
address of DAD D opcode on next line
DAD D ; Perform relative address calculation
POP D ; Pop old HL value from Stack
PCHL ; Jump to new relative location
And if you need a conditional relative jump, you can do:
LXI D, branch_offset ; Calculated relative to the DAD D
opcode below
CALL WHEREAMI ; 31E9H or 3F3DH. HL now contains
address of DAD D opcode on next line
DAD D ; Perform relative address calculation
POP D ; Pop old HL value from Stack
JZ (or JNZ, etc.) 0ED7H ; Conditional jump to a PCHL opcode
; Fall though if condition not met
Ken
