folkert/simh-testsetgenerator
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

AltairZ80: Fix head stepping to only occur on 1 to 0 transitions

Browse source 
The source file to be updated provides emulation of the North Star DD
floppy disk controller. The proposed change steps the head only when the
step bit in the order byte transitions from 1 to 0. This is the way the
actual hardware functions. Previously, the emulation would step any time
an order was issued and the step bit was 1. This behavior caused some
original software (e.g., Lifeboat CP/M 2.2) to end up on an incorrect
track.
This commit is contained in:
Mike Douglas 2015-08-08 21:27:09 -05:00
parent bb39ce97c7
commit f3af74e444
1 changed files with 7 additions and 1 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

8
AltairZ80/s100_mdsad.c
View file

@ -101,6 +101,7 @@ typedef struct {
uint8 ss; /* Specifies the side of a double-sided diskette. The bottom side (and only side of a single-sided diskette) is selected when SS=0. The second (top) side is selected when SS=1. */ uint8 ss; /* Specifies the side of a double-sided diskette. The bottom side (and only side of a single-sided diskette) is selected when SS=0. The second (top) side is selected when SS=1. */
uint8 dp; /* has shared use. During stepping operations, DP=O specifies a step out and DP=1 specifies a step in. During write operations, write procompensation is invoked if and only if DP=1. */ uint8 dp; /* has shared use. During stepping operations, DP=O specifies a step out and DP=1 specifies a step in. During write operations, write procompensation is invoked if and only if DP=1. */
uint8 st; /* controls the level of the head step signal to the disk drives. */ uint8 st; /* controls the level of the head step signal to the disk drives. */
uint8 pst; /* value of step signal (st) on previous order */
uint8 ds; /* is the drive select field, encoded as follows: */ uint8 ds; /* is the drive select field, encoded as follows: */
/* 0=no drive selected /* 0=no drive selected
* 1=drive 1 selected * 1=drive 1 selected
@ -312,6 +313,8 @@ static t_stat mdsad_attach(UNIT *uptr, char *cptr)
if(mdsad_dev.units[i].fileref == uptr->fileref) { if(mdsad_dev.units[i].fileref == uptr->fileref) {
break; break;
} }
mdsad_info->orders.st = 0; /* ensure valid state */
} }
/* Default for new file is DSK */ /* Default for new file is DSK */
@ -510,6 +513,7 @@ static uint8 MDSAD_Read(const uint32 Addr)
mdsad_info->orders.dd = (Addr & 0x80) >> 7; mdsad_info->orders.dd = (Addr & 0x80) >> 7;
mdsad_info->orders.ss = (Addr & 0x40) >> 6; mdsad_info->orders.ss = (Addr & 0x40) >> 6;
mdsad_info->orders.dp = (Addr & 0x20) >> 5; mdsad_info->orders.dp = (Addr & 0x20) >> 5;
mdsad_info->orders.pst = mdsad_info->orders.st; /* save previous state of st */
mdsad_info->orders.st = (Addr & 0x10) >> 4; mdsad_info->orders.st = (Addr & 0x10) >> 4;
mdsad_info->orders.ds = (Addr & 0x0F); mdsad_info->orders.ds = (Addr & 0x0F);
@ -543,7 +547,9 @@ static uint8 MDSAD_Read(const uint32 Addr)
/* use latest selected drive */ /* use latest selected drive */
pDrive = &mdsad_info->drive[mdsad_info->orders.ds]; pDrive = &mdsad_info->drive[mdsad_info->orders.ds];
if(mdsad_info->orders.st == 1) { /* step only on transition of the step bit from 1 to 0. This duplicates the
way the hardware functions and is required by some North Star code */
if((mdsad_info->orders.st == 0) && (mdsad_info->orders.pst != 0)) {
if(mdsad_info->orders.dp == 0) { if(mdsad_info->orders.dp == 0) {
sim_debug(SEEK_MSG, &mdsad_dev, "MDSAD: " ADDRESS_FORMAT sim_debug(SEEK_MSG, &mdsad_dev, "MDSAD: " ADDRESS_FORMAT
" Step out: Track=%d%s\n", PCX, pDrive->track, " Step out: Track=%d%s\n", PCX, pDrive->track,