Retro/pcjs
Watch
1
0
Fork
You've already forked pcjs
 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

Fit ENHDEBUG in /disks/pc/tools/misc on a 360Kb disk, using: diskdump --path="enhdebug/archive/DEBUG.COM;DEBUG.TXT;README.TXT;HISTORY.TXT;LICENSE" --label=ENHDEBUG --output=enhdebug/ENHDEBUG.json --format=json --overwrite --manifest

Browse source
This commit is contained in:
Jeff Parsons 2016-04-30 11:13:28 -07:00
commit 80879eb7af
4 changed files with 15 additions and 28 deletions
Download patch file Download diff file Expand all files Collapse all files

10
devices/pc8080/machine/test/README.md
View file

@ -26,11 +26,11 @@ The good news: PC8080 passes all the 8080 Exerciser tests. And it doesn't do it
Like all the other CPU emulations in the PCjs Project, PC8080 never "calculates" the flags unless/until they are
actually required, which considerably speeds up all arithmetic operations.
Of particular note are the 8080's subtract,
compare, and decrement operations, which actually perform addition, not subtraction, by using two's complement
arithmetic in "stages": the first stage (inverting the source operand) occurs *before* the addition, and the second
stage (incrementing the inverted operand) occurs *after* the addition. And it appears to be the result of the *first*
stage, not the second, that determines the state of the Auxiliary Carry flag (AF).
Of particular note are the 8080's subtract, compare, and decrement operations, which actually perform addition,
not subtraction, by using two's complement arithmetic in "stages": the first stage (inverting the source operand)
occurs *before* the addition, and the second stage (incrementing the inverted operand) occurs *after* the addition.
And it appears to be the result of the *first* stage, not the second, that determines the state of the Auxiliary
Carry flag (AF).
At 2Mhz, the 8080 Exerciser tests take quite a while, but you can click the speed button while the machine is running
to increase the simulated speed; it will revert to 2Mhz when you exceed the maximum speed that your system supports.